Which version of Overachiever are you using?
v0.46

What steps will reproduce the problem?
1.  Opening the achievement panel for the first time after logging into a character

What is the expected output? What do you see instead?
The panel opens fine, but a bug message (I have Swatter installed) pops up.

Do you have an error log of what happened?
Yes.  See below.

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).
No.  It only occurs when "MoveAnything! WotLK" is installed.  Version 3.0.2-8.   They haven't updated since November 2008 so I'm more hopeful you can solve the issue.  Both of your mods are very useful.

Please provide any additional information below.  
Date: 2009-08-23 16:03:11
ID: 1
Error occured in: Global
Count: 1
Message: ..\AddOns\Overachiever\Overachiever.lua line 973:
   attempt to index field 'AchievementFrame' (a nil value)
Debug:
   [C]: ?
   Overachiever\Overachiever.lua:973
      Overachiever\Overachiever.lua:776
   [C]: LoadAddOn()
   ..\FrameXML\UIParent.lua:229 UIParentLoadAddOn()
   ..\FrameXML\UIParent.lua:299 AchievementFrame_LoadUI()
   ..\FrameXML\UIParent.lua:349 ToggleAchievementFrame()
   [string "TOGGLEACHIEVEMENT"]:1:
      [string "TOGGLEACHIEVEMENT"]:1
AddOns:
  Swatter, v5.5.4293 (WombatII)
  AckisRecipeList, v1.0 3.2 Beta 2 2323
  ArenaLog, v1.09
  AtlasLoot, vv5.07.05
  AtlasLootBurningCrusade, vv5.07.05
  AtlasLootCrafting, vv5.07.05
  AtlasLootOriginalWoW, vv5.07.05
  AtlasLootWorldEvents, vv5.07.05
  AtlasLootWrathoftheLichKing, vv5.07.05
  AtlasLootFu, vv5.07.05
  AucAdvanced, v5.5.4293 (WombatII)
  AucFilterBasic, v5.5.4293 (WombatII)
  AucFilterOutlier, v5.5.4293.2531
  AucMatchUndercut, v5.5.4293.2531
  AucStatClassic, v5.5.4293 (WombatII)
  AucStatHistogram, v5.5.4293 (WombatII)
  AucStatiLevel, v5.5.4293 (WombatII)
  AucStatPurchased, v5.5.4293 (WombatII)
  AucStatSales, v5.5.4293.2842
  AucStatSimple, v5.5.4293 (WombatII)
  AucStatStdDev, v5.5.4293 (WombatII)
  AucStatWOWEcon, v5.5.4293.2530
  AucUtilAHWindowControl, v5.5.4293.3311
  AucUtilAppraiser, v5.5.4293.2530
  AucUtilAskPrice, v5.5.4293.3175
  AucUtilAutoMagic, v5.5.4293.3142
  AucUtilCompactUI, v5.5.4293.2530
  AucUtilEasyBuyout, v5.5.4293.3583
  AucUtilFixAH, v5.5.4293 (WombatII)
  AucUtilItemSuggest, v5.5.4293.3108
  AucUtilPriceLevel, v5.5.4293.2545
  AucUtilScanButton, v5.5.4293.2530
  AucUtilScanFinish, v5.5.4293.2530
  AucUtilScanProgress, v5.5.4293.2530
  AucUtilSearchUI, v5.5.4293.3655
  AucUtilSimpleAuction, v5.5.4293.0
  AucUtilVendMarkup, v5.5.4293.2530
  AuctionProfit, v30200-1
  AutoRepair, v
  Babylonian, v5.1.DEV.130
  Bagnon, v2.9.3
  BagnonForever, v1.1.2
  BagnonTooltips, v
  Bartender4, v4.4.0
  BeanCounter, v5.5.4293 (WombatII)
  CandleTracker, v1.0
  Configator, v5.1.DEV.130
  CrapAway, v
  CTMapMod, v3.006 (CTMod 3.0)
  CurseProfiler, v
  DBMCore, v
  DebugLib, v5.1.DEV.130
  Enchantrix, v5.5.4293 (WombatII)
  EnchantrixBarker, v5.5.4293 (WombatII)
  Gladius, vv1.0.2
  GuardianSpirited, v090813
  Informant, v5.5.4293 (WombatII)
  MillHelp, v1.2.2
  MillHelpGlyphs, v1.2.2
  MoveAnything, v3.0.2-7
  Overachiever, v0.46
  PoMTracker, v2.63
  PowerAuras, v2.6.0
  PreformAVEnabler, v2.41
  RatingBuster, v
  Recount, v
  sct, v6.2
  SilverDragon, vv2.3.2
  SlideBar, v5.5.4293 (WombatII)
  SpellAlerter, v2.05
  Stubby, v5.5.4293 (WombatII)
  ThereIsNoCost, v1.2.0
  XPerl, v3.0.5f
  XPerlArcaneBar, v
  XPerlParty, v
  XPerlPartyPet, v
  XPerlPlayer, v
  XPerlPlayerBuffs, v
  XPerlPlayerPet, v
  XPerlRaidAdmin, v
  XPerlRaidFrames, v
  XPerlRaidHelper, v
  XPerlRaidMonitor, v
  XPerlRaidPets, v
  XPerlTarget, v
  XPerlTargetTarget, v
  XPerlTutorial, v
  (ck=979)