This bug occurred when I made a quick retry with autoplay mod enabled. Instead of starting at 78.9% of the song, it started at the very beginning and sanic'd all the way until the song actually matched the note. Quite hilarious and frustrating at the same time since this scares the crap out of me and my fps drops like crazy, meaning I can't do anything.
osu! version: 20150414.2 (latest)