Which version of Overachiever are you using?
v0.51

What steps will reproduce the problem?
Random (so far)... no matter if any OA window is visible or not. Think it happens often when mouseovering inventory items.

What is the expected output? What do you see instead?
Do not expect any special output, and OA continue working without any problems.

Do you have an error log of what happened?
Date: 2009-12-12 16:36:48
ID: 1
Error occured in: Global
Count: 1
Message: ..\AddOns\Overachiever_Tabs\build.lua line 263:
   attempt to get length of local 'list' (a nil value)
Debug:
   (tail call): ?
   [C]: ?
   Overachiever_Tabs\build.lua:263
      Overachiever_Tabs\build.lua:262
   Overachiever_Tabs\build.lua:328
      Overachiever_Tabs\build.lua:320
   Overachiever_Tabs\build.lua:656
      Overachiever_Tabs\build.lua:649

Does this problem occur when all other addons are disabled? If not, use the process of elimination and report where the addon conflict lies (including the version of the conflicting addon).
Happens too with only OA installed.

Please provide any additional information below.
Spanish esES language pack.