To make sure I have this posted...
Say you have a square tile object in the winter terrset04, OR the summer terrset00, and you load a square tile object from the opposite terrset. It will crash my OE.
My work-around is to select New, which clears the current .til, and go straight to properties and set the terrset for the incoming square tile object. That works.
I can't remember if I've had the same problem creating a terrain tile set. I don't usually switch back and forth too often but it was a pain until I learned what to do.