When the first players started doing the Headless Horseman event in hopes of Sinister Squashlings, they quickly noticed a rather harsh bug. There is about a 50/50 chance when mousing over the pumpkin to summon the Headless Horseman that the game client will lock up and/or disconnect you. Ever since patch 4.0.1 hit the live servers, there have been reports that world items that give the gear icon to let you know it is clickable have been causing a similar crash, and this is likely related to that.
One of the reasons this bug is especially hurting groups is that the people to usually click on the pumpkin are the tanks, who are then disconnected, leaving the groups to fend for themselves until they get back. As a precaution, have a DPSer be the one to click on the pumpkin and allow time to reconnect afterward so that the tank and healer can keep the rest of the group alive. If the fight is over by the time the DPSer gets back, the loot roll and bag of goodies should be ready and waiting.
Update: Orlyia (Blizzard customer service representative) posted a list of known issues and work-arounds for Hallow's End in the customer service forums. They've said that they're aware of the mouse-over issue and the developers are looking into what is causing it.