Error with game

Hello, i’m makign a game for a school assigment and i’ve been getting some wicked error now and the teacher didnt know the answer to it.

The error is that the enemies falling down from the sky is tripping or something until i’ve shot a shot.

The game is that you control a cannon on the bottom and you are to shoot at the falling squidfishies.

The stuff you see in here is the level i’m at.

i would be very thankful if someone would take a look at the attached file and then try to solve it. But DONT reupload a fixed copy, since this is for school, i wanna do it myself.
So if someone would find the error, and then write me what it is with a hint on how to solve it, i would be very thankful.

Thanks in advance

http://www.mediafire.com/?r3esm936c6xu1rd

*EDIT the bug started when i got the score counter to work.