Since this particular kind of bug leads to quite unforgivable crashes, I'll be very specific as to what causes it.
Every time the something other than the player dies, the game registers that kill to an item, usually the one in the player's equipped weapon slot. In the case of melee weapons, since there are special rules regarding which weapon is used (involving Juggler), there's a function that figures out which equipment slot to check. Turns out that, if the melee weapon's firing hook doesn't work, the function returns the torso slot instead! Thus, after the game is over and score stuff begins tallying kills by group, armors are also checked, and since they lack the group property, the game errors. This has been fixed to return a null value, which is appropriately ignored.
Practically-speaking, the error will eventually happen if you're on a challenge that won't let you use melee weapons (as is the case in AoMr, AoSg, and AoPc) and you're carrying a melee weapon around when something on the map died. The only time most players are going to trigger this in such a case is when you pick up the chainsaw in Chained Court and don't unequip it before something is killed: considering there's no reason not to unequip it in AoPc, this would happen quite frequently. To be extra-safe, don't leave the chainsaw (or any melee weapon) in your equipped or prepared slots, and there should be no problem with triggering the crash.