Today I realised it was my poor coding that had created a subltle bug that the test team spotted yesterday.

The Bug

The bug was that Excel itself would crash if there was more than one instance of Excel open with my workbook application in one instance and another workbook application in the other and the user attempted to close my workbook via the grey [X].

The Cause

It turned out to be beacuse I had lazily shown and hidden some form objects rather than creating instances of the forms and opening and closing them and clearing up properly by setting these instances to nothing via a call in the "Workbook_BeforeClose" event when the workbook is finished with.

The solution

Once I re-wrote the form handling to be carried out within a "Dialog Manager" module, to work like a static class in C#, with private fields for each form and public properties to access them, the bug seemed to disappear.

Discussions here indicated it would probably be due to undisposed of form refences being present when the workbook is closed. If you have anything further you can add to this issue, please comment below.