1. I edited a copy of the valve elevator instance
2. I saved it and included it in another map of mine
3. I compiled, it worked. Both maps were opened in Hammer
4. I shut down Hammer. I may have edited the elevator, but I'm not sure. Hammer asked me wether I'd like to save the changes in the elevator map. I hit yes. The dialogue appeared again. I hit yes again. The dialogue appeared again. I hit no.
5. When I re-opened the maps later, both would not open with the error: "C:....\departure_elevaror_bts_base.vmf, line 41: unexpected end of file"
   I deleted my instance and could open my main map again.

I can reconstruct the elevator instance from scratch, but still I would like to know what happened. Did something similar happen to any of guys you once?