Bug: Draw.Number wasn't properly checking that the initial, min and max values passed to it were numbers. On errors Python would set an error flag that would only be caught later on, masking where the real problem was. Pesky bug... Reported by André (jaguarandi) on irc, thanks! PS for bpy devs: PyInt_AsLong returns -1 on error and sets the error flag. With functions like that we need to take special care or not that obvious bugs appear.