Firefox Canvas - Artifacts in path drawing (possible clipping error)

I wrote a Canvas program using some "advanced" graphics features: http://pastebin.com/u5q0W876

It works fine in IE, Chrome and Opera. But in FireFox, it is possible to make an odd artifact appear:

~click in lower right (empty space)

~rotate outer ring (click on it)

~looks like http://img535.imageshack.us/img535/9503/fferr.jpg (see upper right)

I am sure the problem has to do with开发者_运维问答 the function pencilStripes (which draws stripes over the entire screen, which are then clipped to a certain region). It's on line 162 and is used on line 77. But I don't know why FireFox has this error when the clipping works fine on all other browsers.

Adding c.moveTo(0,0) to the end of the pencilStripes function seems to fix it for me. But please don't ask why.