I've done that quite a few times as well.
This is made more problematic by the interface inconsistency where the Tag Panel remains populated with data even when all files are unselected by clicking within the blank area of the file view. When I'm working with several different applications I often click within this area to switch back to Mp3tag, inadvertently unselecting everything. But I don't notice it, because the Tag Panel still shows data. If I then edit something in the Tag Panel and go to save it, only then do I see that the Save button is grayed out, and I lose the edits. When no files are selected, Mp3tag should: 1) show no data in the Tag Panel, and 2) prevent the user from editing any fields in the Tag Panel.
You can see that Mp3tag does clear the Tag Panel if you select everything and then invert the selection with Ctrl-Alt-A. I don't see a reason for not doing this in all cases.
Something closely related that I notice, which makes everything a little more difficult, is that the row highlighting color within the file view changes state to un (or "less") highlighted. Under my current Windows color scheme it goes from a light blue background to a very light gray background whenever focus moves out of the file view area. Why? It makes it less obvious that there are any files selected.