Continuity mistake: There's an electrical outlet on the wall below the window (aligned with the window shade's loop) in Sid's room near the Megadork poster, but when Sid throws Woody to the floor and raises the shade (before he scorches Woody's forehead with the magnifying glass), that outlet has vanished.
Other mistake: When Rex shakes the table causing the baby monitor to fall, the batteries fall out and we see the open battery compartment with markings for positive (+) and negative (-), and space for two AA batteries. Woody jumps off the bed and places the two batteries inside the compartment, but before he closes the compartment note the position of the battery closest to the top of the monitor. The positive side is actually where the negative side should be so the monitor should not work, but the monitor turns on just in time to hear the soldier's warning. (00:13:15)
Deliberate mistake: When "Legs" lowers the Pez duck named Ducky down through the removed lightbulb hole, to ring the doorbell, the lightbulb housing is bigger than the hole but they bring it up through the hole to remove it. (01:03:55)
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.
Other mistake: While Sid's toys are working with Woody to get Scud out of the house, just as Hand-in-the-Box is holding the doorknob we can see the screw mounted on the latch's hasp below the loop - which really doesn't make any sense, but as Hand opens the door the screw has suddenly vanished from the hasp and is now properly mounted on the doorway's loop plate, where it's meant to be. If the screw were really mounted through the hasp, the door could not be opened.
Continuity mistake: All the walls in Andy's room have white wainscoting with the cloud wallpaper on the upper part of the walls, but in the opening shot of the room we see behind the "saloon" box that the cloud wallpaper covers the wall all the way down to the baseboard. The tall dresser isn't where it should be as well. (00:00:40)
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.
Other mistake: There are six various bolts, locks, and latches on Sid's door above and below the doorknob, and some are in their locked positions when the door either opens or closes. For example, after Sid replaces the doll head with the pterodactyl head and calls Hannah, when the door opens we can see the dead bolt and slide latch are both slid out in their locked positions and yet Sid's able to open and shut the door, which is quite impossible.
Continuity mistake: When Buzz is in Sid's sister's room, Woody comes up to it disguised in the Christmas lights. He then leaves them beside the door before he goes in. When they leave the room, the lights are gone. Later the lights return when Woody goes to get them, but there are fewer of them. (00:47:22)
Continuity mistake: When the army soldiers are yelling into the baby monitor that the kids are heading up to Andy's room, the baby monitor is right in front of Woody on the floor. Yet, when the camera pans out to show all the toys scrambling for their places the monitor is gone, and Hamm is right in front of Woody. Then, when the camera zooms back in to show Woody, the baby monitor has reappeared. (00:12:30)
Continuity mistake: Before Woody uses the remote control to have RC push Buzz off the desk, there are red, yellow, and blue pushpins in an empty area of the bulletin board, but after RC hits the bulletin board, in the closeup of the pushpins falling off those colors have changed to green, red, and yellow, with the green pushpin remaining in the empty space, but when the bulletin board starts to fall over that green pushpin is gone and it's back to the red, yellow, and blue pushpins.
Continuity mistake: In Andy's room, the general ("generil") store box under the crib near the closet door has a cow illustration on its right side, but in the next few shots there's a drawing of two windows on that side of the box, and then it keeps switching between the cow and two windows in following shots.
Continuity mistake: In Sid's room, there's a lightbulb hanging over the table (which he turns on when he switches the doll's and pterodactyl's heads), and its electrical cord that hangs beside the lava lamp (filled with toy heads) vanishes and reappears a few times during the scenes there.