Interesting Troubleshooting of a MySQL Crash : filling then freeing the disk

I recently troubleshoot an interesting MySQL crash, and I think it is worth sharing (with the related bugs). MySQL crashed when the disk was full, you can see the free disk graph below. The Y-axis is in the tens of GiB scale and the X-axis is in the hour scale. Can you guess what happened ? Just to make sure we agree on the meaning of the graph above, let's describe it