Random last image in animated gifs

I've had problems using big-bangs "record"-option in my Racket Turtle lib (https://github.com/tyynetyyne/racket_turtle). Sometime the last image of the generated animated gif is one of the earlier images instead of the image corresponding to the final world state.
Here are two animated gifs created by running the exact same code: first one is correct and the second one has a wrong last image: http://1drv.ms/1MQxHLg . The wrong last image appeared about 50% of the time in my testing. These images where produced by running the "bug_hunting.rkt" from this package: https://github.com/tyynetyyne/racket_turtle/archive/master.zip.
I suppose there is something in my Racket Turtle package that triggers this since I was not able to produce it with a simpler animation. Racket Turtle uses only 2htdp/image and 2htdp/universe libs. Tested with DrRacket 6.0.1 and 6.2.

Start by running bug_hunting.rkt from the linked racket_turtle package with the record option, using the 2htdp/image and 2htdp/universe libraries mentioned in the report. Compare repeated GIF outputs and trace how the final world state becomes the last frame; done means the generated animation consistently ends with the final state.