Well I found another one, it took me more than an hour to locate but here it comes;
Read: ID3v2 (I only wanted to see files with the extended v2 tags)
Write: ID3v1 (kinda odd combination butt possible to do)
I discovered this bug by mistake will changing between ID3v1 and ID3v2 look-ups. For each of my folders that I do a clean up, I look first for ID3v1 Comments (Read only ID3v1 tags); clean up, Save, Refresh,... then ID3v2 Comments: clean up, Save, Refresh,... in this sort of jobs of continuos changing between Tags Read/Write options, I had by accident this combination of Read/Write option.
So what happened?
I selected all my files in a folder beginning with A, then Alt-T and remove the tag Comment Itunnorm: then OK. I tought all my A*.mp3 files were cleaned from the tag 'Comment Itunnorm', but NO! I found 34 files all lost complete their ID tags, thats all the files containig ID3v1 tags not only those beginning with A from my original selection! It must has something to do with the tag 'Comment Itunorm' not being supported in ID3v1.