Here's a stumper:

After a period of heavy rains, our toilet actually lifts off the floor. I contacted a local plumbing outfit, and the fellow there assured me that in his many years of experience, he'd never heard of such a thing.

We are in Dallas, TX. Our house is a pier and beam type, built in 1950. It has concrete perimeter grade beams set on piers, and isolated piers supporting wooden beams in the interior. Our soil profile is about 4.5' of black clay (Austin Houston Black), 1' of calcareous clay, then weathered tan limestone for the appreciable remainder.

My theory was that the expansive clay was lifting the soil pipe, and the toilet with it. The total rise approaches 3/8". However, I haven't heard of any neighbors with this problem. It only happens after VERY heavy rains, and it is particularly bad if those rains follow a long dry spell.

Any other hypotheses?