Dirty word, contamination. But sure enough, my tissue culture cells - all the cell lines I started making in January - wound up with bacterial contamination. Ugh. What looked intially like cell debris was on closer examination tiny little bacteria, that made my cells trypsinize and move off their plates.
Five years, no contamination...I started doing tissue culture in June 2003. It's been a loooooong time, so I guess I had it coming, but it's still kind of depressing.