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Okay, nope, I've come back and checked it out:

Did my best to crash the game only using rats, summoners and funny bones (no skulls)

Turns out, for this set up even if my rats get shot and the row overflows, the game can handle it.

So... whatever error is going on, it appears to ONLY apply to Skulls with Funnybones on them.

Not sure why that is, but hope it helps.

EDIT: (That, or there's something special about dude's being shot in row one as opposed to row three... but this seems less likely to be the cause.)