Yesterday I found a few old CD-Rs in my archive with very old MP3s. Last modified in 2003 - I don't even remember which program I used back then. Definitely pre-iTunes … maybe Winamp? Did Winamp have an option to edit tags?
Anyway, I loaded them into Mp3tag, and to my surprise it refused to read the ID3v2.3 tag of about 80% of the files.
I double-checked with a few other programs (MusicBrainz Picard, AudioGuru, ExifTool), and none of them complained.
I'm not saying that this is a bug in Mp3tag, it might well be that the tags are broken. I'm just curious what Mp3tag does not like about them that doesn't seem to bother the others. If anyone wants to have a look, I've uploaded the smallest of those files: id3v2_testfile_bad.mp3 (ignore the login prompt, just click "download only").
In addition and specific to Mp3tag, I'm stopping at a "frame" with ID MP3e (invalid frame ID, prefix of repeated garbage data MP3ext V3.3.17(ansi) in the ID3v2 tag structure.
Aaaaahh ...
"MP3ext" rings a bell ... it was a shell extension to access the tags directly from Windows Explorer. So that's the culprit. No longer needed, since this is now a built-in function. The repeated garbage was probably an attempt to write some padding, so that the tag could be extended later without rewriting the whole file.
Those CDs are mostly ancient backups of files that are still in my audio library and have long been tagged properly, there's no need to salvage them. But cut & paste indeed seems to fix it.
PS, for the records: Overwriting the "MP3ext …" garbage with null bytes, as the standard requires – "The value of the padding bytes must be $00" – made the file readable for Mp3tag, too.
- I don't even remember which program I used back then. Definitely pre-iTunes … maybe Winamp? Did Winamp have an option to edit tags? 
