I haven't lost a neg in a long time...but there was the time when the tissue and 11x14 neg stuck together like glue (I foolishly did not put a fan on to blow cool air over the glass of the contact printing frame). This was for a class demo. And as I stood in front of the class trying to pull the two apart, the 11x14 neg tore in half. I did not know they could do that!:o Very dramatic! Fortunately, I had used the lesser of the two almost identical negs for the demo!
PS...I now know of other ways to protect the neg, such as using a very thin sheet of clear mylar between the neg and tissue.