What steps will reproduce the problem?

I haven't defined the exact step to reproduce the problem, it happened randomly when I was in orgrimmar.

What is the expected output? What do you see instead?

An error window appeared, but pitbull did continue to work normaly.

What version of the product are you using?

I use PitBull V4.0.0-beta4

Do you have an error log of what happened?

Date: 2009-08-10 14:49:29
ID: -1
Error occured in: Global
Count: 1
Message: ...Bull4\libs\LibDogTag-Unit-3.0\LibDogTag-Unit-3.0.lua line 210:
   attempt to index local 'guidToUnits_oldGuid' (a nil value)
Debug:
   [C]: ?
   ...Bull4\libs\LibDogTag-Unit-3.0\LibDogTag-Unit-3.0.lua:210
      ...Bull4\libs\LibDogTag-Unit-3.0\LibDogTag-Unit-3.0.lua:201
   ...Bull4\libs\LibDogTag-Unit-3.0\LibDogTag-Unit-3.0.lua:413 func()
   ...erface\AddOns\PitBull4\libs\LibDogTag-3.0\Events.lua:267
      ...erface\AddOns\PitBull4\libs\LibDogTag-3.0\Events.lua:260
AddOns:
  Swatter, v5.5.4293 (WombatII)
  Bartender4, v4.4.0
  BigWigs, v2.0
  Cartographer, v2.0
  CartographerBattlegrounds, v2.0
  CartographerCoordinates, v2.0
  CartographerFoglight, v2.0
  CartographerGroupColors, v2.0
  CartographerGuildPositions, v2.0
  CartographerInstanceLoot, v2.0
  CartographerInstanceMaps, v2.0
  CartographerInstanceNotes, v2.0
  CartographerLookNFeel, v2.0
  CartographerNotes, v2.0
  CartographerPOI, v2.0
  CartographerWaypoints, v2.0
  CartographerZoneInfo, v2.0
  DebugLib, v5.1.DEV.130
  FuBar, v
  FuBarDurabilityFu, v2.11
  FuBarMoneyFu, vv1.2.1
  Grid, v1.30200.2009080501
  LibRock10, v
  MagicRunes, v1.0.63
  MagicRunesIconDisplay, v1.0.63
  MagicRunesRuneBars, v1.0.63
  Omen, v3.0.8
  PitBull4, vv4.0.0-beta4
  Recount, v
  SlideBar, v5.5.4293 (WombatII)
  Stubby, v5.5.4293 (WombatII)
  (ck=338)

Please provide any additional information below.