Thanks tobe,
I think I see what you mean - I altered the code a little to safe guard against that happening. Though I couldn't recreate it and it may have just been frames getting chopped during the video capture.
Your comment did make me alter the closing animation though - it has good impact when you miss an object but no so much when you've had something in the beam so I added a dithered first frame (see below)
So for anyone interested these are the tiles used to generate the beam
Spawning loop (12 Frames)

Transition to capture (4 Frames)

Capture loop (8 Frames)

Closing (8 Frames)

These look a little nonsensical out of context (I'll try to find time to upload a little SWF where you can just test the transitions)
Also I'm interested to know what people feel about the screen effects in the video?