In version 1.5, there was a bug where joining servers wouldn't work, since the game would get stuck on the connection screen. This bug was fixed in 2.0, until the servers were shut down to release 2.1.
Since 1.5, there was an exploit that let people spawn monsters to bother other players. This exploit was removed on 2.0, but it was reintegrated using a dll uploaded by a spanish YouTuber. However, this was finally fixed on 2.1, along with the removal of Cheat Engine support.