Lag and Desync Bugs
All bugs in this category are generally caused by lag and game desyncronization.
Town Placement of "hotpoints"
Occasionally the characters ("hotpoints") in town will be in a wildly different location than normal (such as a cow placed right next to Griswold). This is caused by clients arguing over the true position of these hotpoints.
Town Placement of Dropped Items
When dropping items in town and entering the dungeon, upon return to town the items dropped will have sometimes moved, even to impossible locations - such as inside buildings or across the Tristram borders. This is caused by other players entering the game and not receiving the correct information as to town placement, so the entering client "moves" all the items around (as long as no other clients are in town).
Disappering Items
Occasionally an item will simply disappear when a character tries to pick it up. It is believed to be caused by lag, although there are reports of occurrences in Single Player mode - which may be a wholly different bug. Generally however this is a result of a game desync, when clients do not agree as to the items that were "generated" by dying monsters.
Desynced Levels
When playing with a partner, it is possible to play in a disjointed level (possibly related to previous bug). This occurs when the level layouts are different for each player. This causes strange effects such as watching your partner walk through walls. You may also see completely different monsters. It is highly dangerous to play this way, and both players should leave the level immediately.
Wrong Character status
Characters can sometimes appear dead, permanently stunned, equipment-less, or other ways due to game desyncs when the player changes his/her status, or when suddenly leaving the game.
Player Ghost
If a player leaves the game while in a "trouble" situation (i.e. swarmed by monsters), it may not have a graceful exit. The character's "ghost" remains, the picture of the player. It can be highlighted by another player, even attacked. On other players' screens that can see the ghost, they may see monsters try to attack the ghost.
Exiting Player
When a player is lagged out, or "times out" of a game, the other clients will occasionally see a "Player has left the game" message instead of the time out message.