My gut feel is Xara's checking of degenerate conditions it not so hot.

It usually happens for me when an objects is dragged through a zero width or zero point, i.e., flipping on the fly.
Try shrinking a circle through itself.

I have had occasions when a random artefact is created; it is therefore possible that a negative large position is so created.

Another one is with Blends; I sometimes can get -5 steps.

Acorn