A few years back (1997 or 1998 I think), some engineers at Queen's University designed and built a potato gun. This was not remarkable in and of itself, as this is the sort of thing that happens when design prowess is coupled with boredom in a higher education setting.

When they fired the device, it apparently blasted the potato through a neighbour's fence. Minor damage, I imagine. However, the gun's booming report was mistaken by one of their neighbours for a shotgun blast, and shortly thereafter, the SWAT team raided their house.

I don't believe any charges were laid, but I suspect the "weapon" was confiscated.

(Source: the Queen's Journal, via my dim memory)