What movie sound effects are wildly inaccurate, and what does it actually sound like?

Gun fire sounds inside a room or building.

In movies people are always getting shot with pistols inside of houses and then they have no trouble hearing others speak.

A pistol shot is incredibly loud and inside a room there is a good chance it could cause temporary or permanent deafness.

Another inaccuracy is the sound of a "silenced" pistol shot in movies. Suppressors (silencers) do not silence the gun shot. They lessen the sound but it could still be heard loudly in the next room. (I have used suppressors.)

Everyone's already mentioned gunfire and suppressors. Guns are typically made quieter in movies for plot, so that characters can still talk normally and don't have to act like indoor gunfire even slightly hurt anyone's ears.

I've heard before that movie punching sound effects are made by throwing ground meat against a glass panel. Don't know if that's true, but they definitely don't sound like real punches, which either can sound like a loud slap (if you're hit in a flat soft fleshy area) or a heavy thud (in a dense bony area)

Swords often make sounds in movies when they are being waved around or even sometimes just being held still. Sometimes they sound like vibrating metal, sometimes They make it sound like they are still being pulled from the scabbard even when they're not touching anything but air to amplify its sharpness. A sword slicing through thin air doesn't really make a loud metallic sound.

The sound of someone being punched. It sounds more like a quiet deep thud.

Silencers on guns.

They don't make a "pew" sound like in the movies. They're still damn loud.

Airplanes in WW2 movies. E.g. a C-47 doesn't sound like a small motorcycle like in the movies. It reality it makes a deep rumbling sound.

Space is silent, but there are internal noises in spacesuits and spacecraft. Gravity was reasonably accurate in this regard, at least.

I remember seeing Les Pacte des loups (The Brotherhood of the Wolf) in the theater and thought it was ridiculous that every hit in a fight sounded like thunder and the theater shook when one protagonist was throw to the ground.

I don't want or need to have the teeth rattled out of my head in order to be entertained.

Space explosions.