Some parts of ZIM must've been made in flash, because many objects are movie clips. Some, however, are impossible to see unless it is paused.
The Wettening has a secret movie clip.
Hobo 13 has an alternate canyon sequence if you switch it to the antimatic. (Same audio. Too bad. The art shows ZIM hitting on Crystal, whom he later purposely destroys. She must've rejected him)
GIR: wait if you destroy dib in the past that means he wont ever be your enemy. then you wont have to send a giantrobot back to destroy him - and then he - WILL be your enemy so you will have to send a robot BACK - (head explodes)