Revealing mistake: When Woody falls out of the closet wrapped in the string lights he hears Buzz's voice, and in the next shot as Woody sneaks up to Hannah's room we see Sid's open door and doorway, which looks perfectly normal. However, after Woody rescues Buzz from Hannah's tea party they both stop in the hallway, and there's another shot of Sid's open door and doorway, only this time it's not normal to say the least. Five of the six hasps, bolts, and locks that should be (and normally are) screwed into and attached to the inside of Sid's door (one hasp mechanism, third one down from the top, is attached to the door's frame) are now actually kind of floating there in mid-air right beside the doorway, as if there's an invisible door there. Weird that Pixar artists deliberately did this, but it's bizarrely funny to see.
Revealing mistake: After Sid runs off with Buzz yelling, "To infinity and beyond," when Woody begs Sid's toys on behalf of Buzz and says, "There's a good toy...he's gonna be blown to bits...we gotta save him, but I need your help," in the next wideshot of the bedroom the green carpet's outer edge around the room is bright white below the brown baseboards - Pixar didn't digitally complete it with color.
Revealing mistake: When Woody grabs hold of Buzz and uses his "karate chop action" while they walk backwards to the open door, as Woody says, "Sorry guys, dinner's canceled," in the wideshot of all of Sid's toys pay attention to the chair and table at the right side of the screen. What we're supposed to be seeing is the swivel chair with four legs/wheels (and the red toolbox resting on the seat) that's positioned in front of the table's angled wood two-by-four legs. But we don't see that. What we do see is the swivel chair and table all blended together - the seat and legs of the chair are somehow behind both sets of table legs and the chair's backrest is in front of the table, which doesn't make any sense.