I often get my jam out of the refrigerator, and it looks like this:
That is, all the jam has gone to the top, looking as if the jar has been sitting upside down. However, this happens fairly regularly in both my home and work fridge, so I don't think someone is systematically shifting my jam upside-down and then flipping it back before I retrieve it. :P It doesn't happen every time though. The jar tends to be half to three-quarters full when this happens, but I haven't noticed any further pattern (and I always get standard Smuckers strawberry jam).
I found a very scattered discussion with a variety of hypotheses here. Some of the posters suggest some weird bacteria, but this has happened regularly with different jars of jam, so I'm betting it's not that. My hypothesis is that it's some sort of physics-y effect (something about temperature change with the fridge and pressures?). That said, depending on the answer, Physics StackExchange might turn out to be the wrong place to post this...
Any hypotheses? I'm happy to vary my jam habits to test ideas. :D