Hi Erik
REF. CONTROLS FROM CONTROLS TOOLBAR USED IN A WORKSHEET
You will be aware of the syndrome that when programming a larger application (and making mistakes) after a time Excel seems to "give up" - giving error messages in code that has been running correctly. I usually find that "cleaning" the code by copying to Notepad then back to a new module solves the problem. Sometimes re-making worksheets too.
We also learn to close Excel down occasionally during a heavy coding session, or even reboot. A symptom is that the shutdown takes 15 minutes instead of the usual 1 or 2.
A similar thing seems to happen to happen in this case. This can be more devastating because it tends to make it impossible to open a workbook - not only that, it can happen some time after the application is "finished". My experience with various forums suggests that this is quite common.
It may be that your "luck" is due to using relatively low numbers of controls, and/or having a later version of MS Office. We are still using Office 97 on NT4 here. With 15,000 PCs testing/upgrading seems to take forever.