Yep, bugs...
The reason classic is called classic is because it will be "remastered" version of vanilla.
Blizzard would never downgrade their server code back to "shit" it was. It was unoptimized and buggy as f**k!
Heck, in Vanilla WoW you could exploit a way to log in into anyone's account if you had username. You think Blizzard would allow that? Holy hell no.