I've also had this issue occur regularly (though again, inconsistently). I saw a Steam thread that proposed it was an issue with Macs going between full and windowed mode but I don't think that's it. It can also skewer a Path of the Damned game because you can't change back to PoTD once you are reset to Easy.
Didn't see anything referring to it explicitly in the patch log today... I couldn't get it to happen just now but I'm basically resigned to not playing until it's fixed.