
Visible crew/equipment: After Marion supposedly dies in the truck explosion and Indy is drinking outside at the bar, watch to the left of him. You can see a man wearing a modern t-shirt and jeans walking through the crowd. (00:42:20)
Suggested correction: This is probably the most famous non-mistake of all time. Denim jeans date from 1871. They were first sold by Jacob Davis and Levi Strauss in 1873 and the design hasn't changed much since then! Plain white cotton T-shirts date from 1898 and were first issued by the US Navy to their sailors in 1913. The design caught on immediately and they flew off the shelves. In short, there is nothing at all unusual about a man wearing jeans and a T-shirt in the late thirties.
He's the only person wearing that outfit in Cairo. Every single other person is wearing "traditional" clothing. He's clearly not meant to be in shot.
He could have been one of the few American tourists in the area.
Noman
If that's true, then what about the two guys standing in the door/archway and the one guy who is sitting in the doorway next to them - all three of them don't match the rest of the extras either? (all three are in the background to the left of Indy before the supposed blooper guy appears and walks right in front of them during the scene).
"There is nothing at all unusual about a man wearing jeans and a T-shirt in the late thirties." However there is a good deal unusual about a caucasian man in jeans and a tshirt in cairo in the 1930s. He is entirely out of place.
Unusual does not mean impossible, and in order for him being there to be a mistake it has to be impossible. It isn't. Unlikely is not impossible. This is not a mistake.
That's not how it works. Say someone is wearing a jacket, is off screen for 4 seconds, then cuts back and the jacket is gone. It's technically possible they ripped it off themselves in 4 seconds on a whim, but that's not actually feasible. There's one solitary man in jeans and a T-shirt in Cairo in the 30s, when every single other person on-screen is wearing different clothing. The most likely explanation is it's a screwup.
Trivia: The German submarine is the same sub that would later be used for Wolfgang Petersen's "Das Boot".
Phil C.Suggested correction: The sub wasn't later just used in 'Das Boot', the full-size prop was actually built specifically for its production. Steven Spielberg only rented it for 'Raiders of the Lost Ark', apparently much to the surprise of the German film crew, who one day found their boat 'missing' as it was on loan to Spielberg.
Deadmarsh28Factual error: When Sallah and Indiana are sitting and talking on the terrace, with Cairo in the background, you can see a modern TV antenna over Indy's left shoulder. At the time the movie is set televisions were not available. (00:35:10)
Suggested correction: Where? No antennas at all anywhere.

Continuity mistake: There's a shot of the plane at the beginning, taken from the front, looking down the length of the body, where there's no writing on the side. (00:11:45)
Suggested correction: On a good quality frame the number is visible quite clear. Not visible on a poor resolution frame.

Visible crew/equipment: As they come out of the "mapping room" in the tomb, there is an extra crouched down by the door, apparently hiding. It seems as if it was a quick fix for someone who was where they weren't supposed to be.
Suggested correction: It's a man, who was punched by Indy in a deleted scene. He is lying still and not crouching at all.
Other mistake: In the bar shootout, a man is shot in the head. The bullet hits him in his hat right in the middle of his forehead, and then his hat falls off. But we then see a close-up of his face, and there's no bullet wound in his forehead or anywhere else.
MikeHSuggested correction: Wound easily could be on a hairy part of the head.
Even in a close up a bullet wound would still be visible due to the size of the wound.
Ssiscool

Continuity mistake: When the temple collapses, Satipo is crouching looking at the chaos. Indy starts to run and there's a wide angle of the passage ahead but Satipo is nowhere around. He couldn't have managed to run that distance in half a second.
Sacha
Suggested correction: He had a couple of seconds to run away from that spot.
It's a straight passage so he should be seen somewhere along it and he isn't.
Sacha