^ Best explanation I've read so far.

No you're not! Break it down easier:
Jareth is standing in front of a door that leads to freedom. Jareth always lies.
Sarah is standing in front of a door that leads to certain death. Sarah always tells the truth.
You walk up to them and need to get to freedom. You and they both know that one of them will always tell the truth and one of them will always lie.
If you ask the question "Which door leads to freedom?" Jareth will lie (because he always lies) and point to the door behind Sarah (which leads to certain death). Sarah will tell the truth (because she always tells the truth) and point to the door behind Jareth (which leads to freedom.) So you get both pointing at each others door and it doesn't get you any closer to freedom.
If you ask the question "Which door would the other one tell me leads to freedom?" Sarah will tell the truth and knowing Jareth always lies, would point to the door behind Sarah which leads to certain death. Jareth will lie and knowing Sarah always tells the truth would also point to the door behind Sarah because Sarah would not tell you that the door behind her leads to freedom when it leads to certain death.
Therefore, both of them are now pointing at the door behind Sarah so you know the safe option is the door behind Jareth. Hope that makes sense!?

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Although correctly explained in this thread, it is INCORRECTLY described at the moment in the FAQ section. No mention of asking the one guard what the other would say. FAQ is currently locked so can't suggest a correction there.

You still can't know who is lying no matter what you ask.
This is technically true. But, keep in mind, your goal is to find out which
door
is which, not which
guard
is which. Think about what answer Sarah would get from each guard with the question she asks. She's either asking a truthful person about what a liar would say or asking a liar about what a truthful person would say. The answer's going to be a lie either way.

I'm sorry, I'm too stupid to get it
I had to stop reading after the first point. It made my head hurt

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What do you mean exactly? Do you mean the premise of the riddle can't be taken at face value (especially since the guards explained the rules to begin with)? Or that the riddle is inherently unsolvable on the face of it? Because this is a famous riddle and Sarah's logic was sound.

It's a plothole. You still can't know who is lying no matter what you ask.

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I would just ask an unrelated question of something like, "Am I male?" Of course, then I pick the door of the one that says "Yes."

Based on the scenario in the film it's impossible for her to find the right answer.
One head on the bottom tells her that one door leads to the castle and one door leads to certain doom. The other head on the bottom tells her that they don't know which one is which, to ask the top heads.
She doesn't know if either of them is lying, so she doesn't even know if either door leads to the castle. She can only ask the top heads about where one door leads, so she can't verify this scenario to be true until after she makes her choice. In the event that she can ask multiple questions, she still won't be able to confirm this. I'll get to that later.
If both doors led to doom, the liar would still tell her one of them led to the castle, so she would get the lie either way from her question and she wouldn't know until it was too late.
Next, one of the top heads tells her that she can only ask one question. The other top head tells her one always tells the truth and one always lies.
If head two is telling the truth, then head one is the liar and she can ask more than one question. In that case she can confirm that by asking another question, but that will only prove the first head is a liar, and not whether the second head is true.
Head two could be lying about one of them telling the truth and they both could be liars. Again there is no way to prove that, other than to make the choice and see which one it was afterwards.
*Here's a ***in' spoiler: Everybody dies.

It's a fairly minor quibble really.
I do not think so. This is supposed to be a logical puzzle, and the writers messed it up. The way they did it, it has only an apparent resemblance to the well-known puzzle. The (somewhat ridiculous) assumption in the riddle is that the characters indeed behave in that extreme way to always lie/tell the truth. That assumption is artificial enough, it does not help to make it murky by quietly assuming additional rules.
The way it is, you can only deduce that both characters do not always tell the truth. Because they both agree on the rules, when the red one starts explaining them. Thus it cannot be that one of them always lies and the other one always tells the truth. But that it exactly what the blue one than says: Always. And the red one nods to it, which makes it even worse.
So basically she is screwed. Obviously she did not chose the path to certain death though, which was a stroke of luck.

That messes it up. Because if the guy who said; "One of us always tells the truth, and the other always lies" is a lie, then there's no point at all to what they're doing. Might as well just guess.
I think the assumption is that, the rules of the game are given honestly by both doors BUT every other statement follows the rules of one is honest and is dishonest.
Really, I think it's just something writer's of the film didn't consider when inserting the scene. And if they had, they'd have perhaps had another character explain the rules of the game, or given her advice as to what to do if she met the doors. It's a fairly minor quibble really. As said, just assume the rules to the game are as seen.

That bit does kind of mess it up, slightly. I guess they mean only when they're asked questions, they have a rule of always lying and always telling the truth, but they are still allowed to speak truthfully when explaining the rules (though, it would be even sillier if the truth-teller was the one who explained it). I thought that when I watched it at one point - I think to save explaining this they should have had the lower guards explain the rules - they aren't part of the rules, and don't know the way to the castle, but they would know that the other two either lie or tell the truth.

I actually thought that the last time I watched this. If "one of them always tells the truth and one of them always lies" is a true statement then the guard who said it is obviously the one who always tells the truth.
Once upon a time there was a magical place where it never rained. The end.

Gys, guys. All this assumes that 'one of them always tells the truth and one of them always lies'.
Sarah's mistake was in taking the veracity of that statement for granted. If what the goblin said was true, than he just outed himself as the truthful one. If it wasn't true, she may as well have flipped a coin.
I suppose a smaller-caliber gun would have to fire baby teeth.

The best question is to just ask "Okay, what would the other door say if I asked which way to go?"
Reasoning: If the honest door answers, he'll say "Well, the other guy would've lied to you."
If he's the one that lies, then he'll say "Take this way." which clearly is the one you DON'T want to go through.

She wasn't trying to get out of the labyrinth at that time though

The door she took is the door to the castle. She fell in a hole because it was simply another obstacle, and only ended up in the oubliette because she chose to go down. Had she asked the hands to take her back up, and out of the hole, she would have been able to keep going. T
Exactly!!!! That was what I always thought too!!!!
That Sarah did NOT make the wrong choice, she made the correct one!
Her error was (inexplicably) choosing to go Down instead of Up.
But even in a way, choosing Down was the right way.since Hoggle was already assigned to get her out of there.
If the Oubliette was only meant to be a trap with no end, why did Jareth send Hoggle there to find her? Jareth could have just left Sarah there.
There had to be some way out that Sarah could have found.
I'd say this cloud is Cumulo Nimbus.
Didn't he discover America?
Penfold, shush.

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The door she took is the door to the castle. She fell in a hole because it was simply another obstacle, and only ended up in the oubliette because she chose to go down. Had she asked the hands to take her back up, and out of the hole, she would have been able to keep going. There was only one liar, and she had the correct path - they never said that there would be no traps or anything, Sarah did not ask that. Also, the oubliette was not "certain death" because the hands gave her a choice, so she still had a chance to avoid it, and if she had known to step over that part she could have avoided it herself.

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That's the easiest way of looking at it, the way Sarah phrases it
guarantees
that the answer she is given is a lie.
If she asks the guy who is telling the truth what the liar would say, he will repeat the lie. Because that is honestly the answer the liar would give.
If she asks the guy who is lying what the honest door would say, he'd lie and say the opposite.
Either way the answer given is a lie, and she must chose the other door.