There is a possibility that the digger ran out of time on the exact frame that he became a faller (as namida stated), and the game rendered him graphically as a faller, while still thinking of him as digger, when the next update came in, he became an ohnoer, because as namida said, the digger somewhat "stands" on any of the nine pixels in the destruction mask, but when he becomes an ohnoer he falls, because he can't stand on the pixel that the digger was standing on. This would result in the fake faller becoming an ohnoer, making it seem that the digger actually became a faller, but was actually a digger. That makes this a graphical glitch.
Speaking about graphical glitches, in lemmixplayer, the explosion confetti sometimes stays frozen in the air. But, if another explosion occurs, the confetti disappears.