On both the LL and Open Source Clients, accidentally touching the arrows next to the size edit will reduce a prim to 10x10x10. This is a major bug particularly for people with huge prims for pay.
For the rest of us it is just very annoying. I will search JIRA on Monday, and if no one else has filed this yet, I will.
For me I seem to be triggering it when I hit tab once too many to make fine position adjustments. The best defense is when working with a tortured huge prim or group that requires any sort of time to remake, always make a copy before positioning it.