When the bathtub falls, the chunk of floor it was sitting on--which would have also fallen--is nowhere to be seen.
When the bathtub falls through the floor, Walter is pouring water into it, but when the camera angle shifts to an upward shot from the floor below, no water is being poured.
When Walter goes into the den to call Montgomery Schrapp, a candle is visible on the desk beside the phone. Walter uses a match so he can see the number on the paper, and the paper catches fire. When Anna gets home and comes into the house, she turns on the light. When she finally finds Walter in the floor in the den, she turns on the light in that room too.
When the kitchen's electricity goes haywire, the stove door opens twice: once when it is first stuck; again when the meal explodes out.
When Walter is trapped in the carpet in the hole in the floor, his level varies. Sometimes he's down to his shoulders and other times he's only trapped to the level of his chest.
After Walter and Anna have hauled two two-gallon buckets of water up the ladder and dumped them into the bathtub, Walter goes back downstairs and, after the kitchen fire, brings up the third recently heated on the stove. That is also dumped into the tub, causing the tub to crash through the floor and land below. As the camera angle picks up the tub falling through the floor and the ensuing crash and breakage, there is no water spray from the broken tub.
When Carlos says goodbye to Walter, Sr. and his Brazilian wife, he first says "Auf Wiedersehen!" (German), then tries to correct himself by saying "Hasta la vista." However he is still using the wrong language, as "Hasta la vista" is Spanish and not Portuguese. This can be explained by the fact that Carlos is from Paraguay, so Spanish would be his first language.
When Anna is leaving Max's house after spending the night, she grabs her stockings which are hanging from the fireplace. The stockings are obviously brand new fresh out of the box.
When the raccoon scares Anna, she nearly walks off the second level after the stairs had just collapsed. Walter is hanging where the stairs were and says that Anna is standing on his fingers but she is too far back for that. It's only in the following scene that her feet are overhanging the edge and on Walter's fingers.
When the lead Cheap Girl tells Walter they want to change their name, you hear "Meryl Streep" but his lips say something else (like "bulls**t"); the same for Walter when he tells him he's not changing their name. Perhaps another name was originally used and the producers didn't get the permission to use it.
Pushing the raccoon out of the dumbwaiter.
When Walter's lungs are compressed from falling into the hole with the carpet, trapping his arms, he is unable to shout out to Montgomery Schrapp (plot device).
In the following scene when Anna is searching for him, he is clearly able to shout to her: "Upstairs!!" (full Tom Hanks signature shout).