I was doing an artifact realm and had just warped to the next level (62/100), when I received a “stacker overflow” message. Game went dark and gave a message on screen saying “saving game, please wait” and stalled there until I manually closed the game. I reopened the game and my game save was still there (whew) and my artifact realm progress had saved at 61/100. I do not know what caused it or if it is reproducible.
Version .3.2, Windows 7
Edit: Happened again - text is: