I was in a five-man party. I've never seen this particular error before. This is the error popped up when a new player joined the group:

Message: Interface\AddOns\Recount\Tracker.lua:1185 attempt to perform arithmetic on local 'amount' (a nil value)
Time: 12/09/11 14:00:05
Count: 3
Stack: Interface\AddOns\Recount\Tracker.lua:1185 in function `AddAmount2'
Interface\AddOns\Recount\Tracker.lua:1807 in function `AddDamageData'
Interface\AddOns\Recount\Tracker.lua:579 in function <Interface\AddOns\Recount\Tracker.lua:566
Interface\AddOns\Recount\Tracker.lua:1035 in function `?'
...chy\Libs\CallbackHandler-1.0\CallbackHandler-1.0.lua:146 in function <...chy\Libs\CallbackHandler-1.0\CallbackHandler-1.0.lua:146
[string "safecall Dispatcher[18]"]:4: in function <[string "safecall Dispatcher[18]"]:4>
[C]: ?
[string "safecall Dispatcher[18]"]:13: in function `?'
...chy\Libs\CallbackHandler-1.0\CallbackHandler-1.0.lua:91 in function `Fire'
...face\AddOns\Archy\Libs\AceEvent-3.0\AceEvent-3.0.lua:120 in function <...face\AddOns\Archy\Libs\AceEvent-3.0\AceEvent-3.0.lua:119

Locals: <none>