For some reason the game takes a screenshot when pressing the button that produces the characters "< ,> and |" (which is the button between the left-shift and Z keys) on at least Scandinavian (and probably a few other) keyboard layouts. (Also this is a bug because it doesn't/didn't happen in 1.7.x)
Environment
Probably nothing else except the keyboard layout (and that the MC version that is used is 1.8.x) matters
Duplicates MC-68399