Thanks PKPHaLaNX.

"this is caused by floating point operations. So if you try to bool to small areas, the position of vertices may float to 0 and your geometry is lost"

OK, but so long as they're not integers, why are they floating anywhere?

"this is not a bug...

It's mathematical uncorrectness..."

Well put!

OK, I'm in danger of being a pedant now ;-) , but when floats (or doubles) floor to zero when you'd rather they didn't, this 'annomaly' is usually debugable and correctable.
I don't have 1% of the brain cells of the Maya guys, so I know I'm missing something... I'd just love to be able to understand the reasoning behind this.