All the usual stuff, but sometimes they get it right
One particular instance comes to mind, from the movie Sahara.
In the shootout at the village well, the sidekick character picks up an AK, dropped by a shot soldier. He actually pops the mag, looks at the ammo, reinserts the mag, and chambers a round. I found that a very plausible thing to do, considering that you have no way of knowing if that particular warlord's thug even had ammo in the gun, and AKs don't lock open empty. It was, refreshing.
As has been mentioned, Garands do not always close upon loading.
The .30 cal Browning can be fed by either cloth belts, or disintegrating metal link belts.
The sniper shot through his scope is not BS. It has actually happened, and is documented (Carlos Hathcock - Vietnam). A very rare thing to be sure, but not impossible. The US sniper changing his scope is not believable. Sure, it can be done, and will work on the range, but it wasn't done in combat. Running, diving for cover, and a whole lot of other things will virtually ensure that any scope you carry in your pack will not still be zeroed when you get around to use it. And as far as I know, spare scopes were not issue items in WWII (or any other war). Another goof is the sniper talking about windage, while adjusting the objective lens (parallax).
And about the famous belts of ammo with the missing primers, one movie websight says that the primers (which were there) were black, so it just looked like they were missing. Yeah, right. Did anybody else notice that at least one of those belts of ammo had holes in the brass? This would look like a black spot on the case body, which is one of the two ways the military uses to ID dummy ammo.
The sounds that guns make are ALL added in, post production. ALL the gunshots, hammer clicks, slide racking and empty guns going click, click, click, are all added in after the movie is in the can. It isn't the actors, and often it isn't even the directors, it is the foley artists (the sound guys) that do this during the editing process. They add in nearly all the sounds other than the actors dialog. Doors slamming, stairs creaking, tires squealing, spaceships "zooming" through airless space (where sound cannot travel), all of it. This is done because of the fact that for most situations the background noise would drown out the actors voices if recorded at "normal" levels. And as one person noted, the stupid sounds of things that we know don't happen (like an M16 or a Glock going click, click, click when empty), Hollywood knows that everyone knows that when a gun is out of ammo it goes click, so they put that sound in so that the viewer will know the gun is out of ammo. It doesn't matter if the gun would really make that sound or not.
