Me and a few mates have always wondered who's foot we see in Aliens during the scene when the Aliens first attack the Marines inside the atmosphere processor. We see Frost get torched by Deitrich's flame thrower and he falls down a stairwell,just at the very end of the shot a foot comes into view from the left handside for a few frames. My speculation is it's a crew member waiting to douse the flames on the FX dummy after the shot. But it's one of those things now that we can't help but spot everytime we watch the film !!