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.