This evening, I was working on rebuilding my sister's Windows XP machine because it became infected with some fake, rogue, antivirus malware. I launched Microsoft Update from the Start menu (really just an Internet Explorer shortcut that calls "C:\WINDOWS\system32\rundll32.exe C:\WINDOWS\System32\muweb.dll,LaunchMUSite") and I got the following totally esoteric and unhelpful error message, "The requested lookup key was not found in any active activation context.":
Oh of course! Silly me, always trying to request lookup keys that are not found in any active activation context. What was I thinking?