This happened last week and I put it down to me not paying attention to what I was doing, but it's just happened again and this time I was 100% with it....
I've just merged the patch of all my house light channels from an old show file and after the merge had completed I was left with an 'untitled*' show file and just my house light channels. Everything else had gone.
I re-loaded from the save I had done before the merge and tried again and it worked fine. Pretty sure it wasn't me both times.
Andrew