It means you or its author poorly coded the macro or add-on that YOU
installed. You'll have to isolate which add-on is causing the problem.
Start Outlook in its safe mode ("outlook.exe /safe") and retest to see if
the error still occurs (but under what conditions only you know since you
didn't divulge any details here). If that works, disable all add-ons and
reload Outlook in its normal mode and retest. Then start enabling the
add-ons one at a time and retest until the problem returns at which point
you'll know which add-on is causing the problem.