On the Lost Isle while doing the quest ‘The Pride of Kezan’ you cannot complete it. The minute you get into the plane to take off the game crashes and kicks you out. I’ve done all the troubleshooting available and cannot complete the quest and leave the Lost Isle to continue to complete my character. This is on Mists of Pandaria Classic. So for now My character gets to sit there and wait. I’ve reported the bug as well.
In my mind a crash and a disconnect are separate issues so please clarify. Is it a crash with a box and a long code, or a disconnect with a code like WOW51900319?
Looking it up on wowhead seems to show using the space bar causes the disconnect, but that might have been fixed? Not sure.
For classic specific bugs, you can also post in WoW Classic > WoW Classic Bug Report