The energy bill that Reznik opens, 20 minutes into the film, shows the unlikely meter number of "123-456789". Presumably it was hoped this would not be spotted with the distraction of the large FINAL NOTICE above it.
The goof items below may give away important plot points.
When playing the game "Hangman", if a correct letter is guessed, all appearances of that letter in the word get filled in. However, about halfway through the movie we see the "Hangman" post-it note with only one 'L' filled in, even though the final answer had two 'L's in it.