I found out about this bug today, My laptop was having problems so i had to disconnect in battle. When I rejoin the NPC's dont attack me at all, They where attacking my pet and friend only. We went and tested this and entered another battle. Once more I disconnected by hiting the X button and same thing happend. We tested it once more and the third time this happened agean. I thought i should bring this up to be fixed..Its a big bug i found due to my PC crashed GO VISTA!!!
Most likely when you disconnected, it reset your aggro.
I'm not entirely certain how the aggro system works, but I think it's something like this. When a mob joins a battle, it sets a random aggro value to all players (and minions) already in the battle, generally no more than a few hundred damage points worth. When another player joins a battle late, either mobs don't assign any new aggro to the other player, or else the aggro value they assign is capped at much lower.
Dealing damage to a mob does some aggro to that mob. In other games, healing and buffing often does some aggro to all mobs, but I'm not sure if that happens here. Regardless, mobs attack whichever opponent has done the most aggro to them.
Thus, if another player joins a battle partway through, previous mobs won't attack that player initially. This is most visible with minions that you summon partway into a battle. Mobs that join later will still set aggro values for all players already in the battle as normal.
I have been seeing in long boss fights only , where you get the linking disconnect go yellow then the fight restore, however you cannot see your shields debuff pips, etc but you potion bottles become available. you don't see the casting on easier side only the animation if the casting goes through or sometimes the monster don't attack, and just keep time-out passing. I too am on vista and both I have had this in the last week, I did not show any issues with my connection to the internet, my computer is the dmz also so all ports are forward from router, and my computer personal firewall has all the port open for wizards 101, it does seem to be something on the client end and possible vista though.