WowAce

What steps will reproduce the problem?
1. Log on
2. Enter the heroic Azjol-Nerub instance.
3. Mouse over the first boss (Krik'thir the Gatewatcher) and his surrounding mobs before going into combat.

What is the expected output? What do you see instead?
Expected: no problems. Instead: error is thrown up which is repeated nine times.

What version of the product are you using?
r66

Do you have an error log of what happened?
MobSpells-r66\\MobSpells.lua:453: table index is nil\nCallbackHandler-1.0-3 (Ace3):146: in function <...Ons\\Ace3\\CallbackHandler-1.0\\CallbackHandler-1.0.lua:146>\n<string>:\"safecall Dispatcher[1]\":4: in function <[string \"safecall Dispatcher[1]\"]:4>\n<in C code>: ?\n<string>:\"safecall Dispatcher[1]\":13: in function `?'\nCallbackHandler-1.0-3 (Ace3):91: in function `Fire'\nAceEvent-3.0-3 (Ace3):119: in function <Interface\\AddOns\\Ace3\\AceEvent-3.0\\AceEvent-3.0.lua:118>\n\n  ---",
            ["type"] = "error",
            ["time"] = "2009/04/15 18:57:59",
            ["session"] = 918,
            ["counter"] = 9

Please provide any additional information below.
USEnglish client/server, WoW 3.1 live, standalone fully updated libs. After this error occurred, the tooltip still seemed to provide MobSpells data.

Azjol-Nerub heroic--and only getting up to the first boss before the server crashed--is the only instance have been able to play since the patch. The instance servers have been regularly crashing on my realm, so haven't been witness to more errors yet.

The above identical error has since happened in UK heroic, UP heroic, OS heroic & Uludar. It repeats many times each run (Uludar tonight had it repeat 44 times).