During IR training I was flying somewhere at around FL70 or something like that, in the middle of cloud, but nice and smooth and calm.
Was getting a bit thirsty so unscrewed the cap of my water bottle and went to take a swig.
Of course that's the point the cloud decided to get bumpy. Drop my water bottle on my lap and proceed to lose the entire 500ml contents of cold water right into my crotch. Wasn't impressed. Instructor found it hilarious.
After that you tend to be somewhat careful with drinks when flying.