So this showed up:

Date: 2008-07-20 16:12:08
ID: 4
Error occured in: Global
Count: 1
Message: SendChatMessage(): Chat message must be UTF-8 text
Debug:
   [C]: ?
   [C]: ?
   [C]: ?
   [C]: SendChatMessage()
   [string "function Nx.returntrue:ifwtfwtf(breaklocald..."]:1: ifwtfwtf()
   [string "function Nx.returntrue:niland(iftrue) local..."]:1: niland()
   [string "function Nx:NXOnUpdate(iftrue) local Nx=Nx ..."]:1: NXOnUpdate()
   [string "*:OnUpdate"]:1:
      [string "*:OnUpdate"]:1
AddOns:
  AtlasLoot, vAtlasLoot Enhanced v4.05.00
  AucAdvanced, v5.0.PRE.3117
  AucFilterBasic, v5.0.PRE.3117 (BillyGoat)
  AucScanData, v1.0
  AucStatClassic, v5.0.PRE.3117 (BillyGoat)
  AucStatPurchased, v5.0.PRE.3117 (BillyGoat)
  AucStatSimple, v5.0.PRE.3117 (BillyGoat)
  AucStatStdDev, v5.0.PRE.3117 (BillyGoat)
  Auctioneer, v5.0.PRE.3117
  Carbonite, v1.72
  EnhTooltip, v5.0.PRE.3117
  Gatherer, v3.1.2
  Informant, v5.0.PRE.3117
  LightHeaded, v239
  LightHeadedDataD, v239
  LightHeadedDataE, v239
  Omen, vOmen r72207 / Threat-2.0 r71636
  Stubby, v52
  Swatter, v5.0.PRE.3117
  TrinketMenu
  Wardrobe
  DBMAPI

Good luck with it :)

Have looked for that several times before. Cannot find the cause. It seems to be a rare error.

Made the change below for the next version that will allow someone with the message to send me the bytes and hopefully I can see what data being sent has a bad value. 

Fixed chat UTF-8 error message. Dumps error to chat window instead.