Too many items in view

This was, again, a not-so-advanced problem, but it appeared quite unexpectedly and actually shouldn't even be possible!

Context

A couple of years ago, I was working on a system for measuring mobile radio transmission parameters. It was originally written in Microsoft's Microsoft Foundation Class (MFC) framework and we were porting it to Qt and adding some new capabilities along the way. As the system was quite complicated, the UI was rather advanced too, but was working quite decently.

Problem

The only problem that manifested itself from...