The Bodyguard.
Remember the microphone and the sound guy in the doors reflection?

The gun Indy uses in bar fight in "Raiders of the Lost Ark" switches very obviously between a revolver to an automatic to back again several times. It makes no sense.

Duel (1971)
David Mann leaves Chuck's Cafe and walks to his car. The driver's door is wide open when he had clearly closed it before going inside.
It was probably a mistake but I like to believe the truck driver opened it to look in his car.
Just because I'm not on THEIR side, doesn't mean I'm on YOURS.

Dendrolagus ursinus

marky mark's fake dick at the end of boogie nights
I crushed the pathetic loser troll Cuck_Venom and rebuilt him as my toilet

Don't know if you'll allow something from a "Blue" movie, but here goes:
Early 70's I go to my first
XXX film
. There is a scene where a cop is doing it to a girl. The scene is shown from two angles which they keep switching between.
The problem was, in one angle the cop is on top and in the second angle the girl is on top. So every few seconds they suddenly flip positions (without breaking stride, either.)
I just about died laughing.

The best I've seen was in The Fifth Element when Ian Holm's character actually calls Bruce Willis's character "Mr. Willis"

The entire Oceans 12 movie is a goof. The plot is for the gang to steal a Faberge Egg from a museum display before their rival thief can do it. They put on a huge production, including Julia Roberts pretending to be a pregnant "Julia Roberts." They create a hologram projector. They bring in outside actors. It goes on and on with a ridiculous heist plan.
Then they show us at the end that they actually stole the egg while it was being transported to the museum. In other words, they already won the bet.
What the hell was the rest of the movie about?

A really bad goof (if you can call it that) was in Ed Wood's "Bride of the Monster", where Bela Lugosi's pet octopus finally turns on him. Well, the director forgot to get the battery that allowed the rubber octopus to come alive and actually look like it was actually attacking Bela, so Wood told Bela to climb on top of the octopus and move its tentacles around and throw himself around and act distressed. You can totally tell it's fake. It is obvious and completely ridiculous. And it's my personal favorite "bad" moment in an Ed Wood. It is hilarious.
I thought I was gonna die! - Roseanne Roseannadanna

Julia Roberts starting breakfast with a croissant and in the next shot she's eating a pancake.

I don't have time to see if this came up already, but for me it was when Princess Leia yelled "Luke!" near the end of STAR WARS and he replied, "Carrie!"

Not sure what the most obvious one is to other people, but a really obvious one I noticed recently is in "drag me to hell".
At the end when the main character is digging into the grave, the hole is tiny in one shot and massive in the next one. Probably cut for time, but still glaringly obvious.

I agree on this one as I watch it often and laugh every time. In my opinion it was a terrible camera angle and should have been easily caught.
There is another one that I saw and nobody I know mentions. In the scene where Michael is having dinner with Kay in the hotel room the levels in the wine glasses keep changing. Again, it is particularly noticeable due to the camera angle. Definitely one that can be faulted to the continuity team.

This may be in here already. This one is so bad because it's in a great movie. When Sonny goes after Carlo in The Godfather, and Carlo has his back against that brick wall, Sonny throws a right cross and it misses hitting Carlo by 2 or 3 feet, but Carlo swivels his head around like he just got hit by GGG.
That is the worst pulled punch I've ever seen in a movie.
There is also that famous Goof in Bullit starring Steve Mcqueen. The famous car chase where Steve is in the green Mustang. At the end of the chase, the other car is supposed to hit the gas pump but it goes wide right of it, yet it still explodes (the gas pump at the gas station.)
I read the scene was so expensive to shoot, they just left it that way & the producers didn't want to pay to do it again. When the movie came out, nobody noticed it and the film did very well for The King of Cool at the box office.

Gim Carry

Star Wars Darth Vader didn't realize Luke Skywalker was his son despite his last name being SKYWALKER.
Show me the holes!

I think, in my opinion, the most obvious goof ever would have to be in the movie Troy, with Brad Pitt. An airplane is accidentally caught in the background during it's takeoff! Why this goof was overlooked by the film crew and never edited out, I will never know! Has anyone else ever noticed this mistake before?

"White Lightning". The first time I ever noticed a goof and it was blatant.
Even as a non-driver I picked up on the fact that Gator was driving a car with a manual transmission with stick shift on the floor but when he arrived home the car had morphed to an automatic with gear shift on the steering column.

In Goin' South, Jack Nicholson is about to be hanged on a gallows without a trap door.
This post has not been deleted.

One that really manages to stand out for me is in 'The Maltese Falcon' when the shadow of the boom-mic is clearly/obviously shown moving over Elisha Cooke's body as he's walking down the hall with Bogart.
"Life IS pain highness. Anyone who says differently is selling something".

Not of all time, for sure, but James Spader as Raymond Reddington using the word "jejune" rather than "blase" was pretty lame. The Blacklist Episode: Marvin Gerard S03, E02
I guess the writers were trying to make him look intelligent by using a word they're guessing no one else knows the meaning of. Or they're dumb and/or unlettered writers. These days? It's more than likely. Anyway, yawn.

Charlton Heston denies the red car in the audio commentary of the DVD. He also said no one was killed shooing the scene. (Another legend.)
This post has not been deleted.

And I'm surprised nobody has mentioned the little red car that's visible during the chariot race in Ben Hur.
I've watched that scene dozens of times and have never seen the legendary red car, which I have also read is a myth. Do you have a more specific description of where and when to find it?

In
Breakdown
, there's a scene where Kurt Russell's character and another guy (who is holding him hostage) stop their car at a railway crossing, and after a brief scuffle they continue driving, only now the railway crossing and the railway are gone and its just a straight, flat bit of road.
In
Cat Ballou
, both the musicians who are partly narrating the film are clearly not playing their instruments. Also a small Cessna can be seen in the scene where Lee Marvin is shooting sticks for target practice.
In
Con Air
, some prisoners break open a box of shotguns and throw them to other prisoners. When these prisoners catch them, the shotguns are now M16s. The shotguns are never seen again.
John Carpenter's The Thing
. When MacCready records his short message onto a tape, he mentions that Windows found some bloody and torn long johns with the name tag missing. It was actually Nauls that found them.
And I'm surprised nobody has mentioned the little red car that's visible during the chariot race in
Ben Hur
.
They call me the wanderer.