It happens from time to time that I accidentally overwrite APEv2 tags which store the MP3Gain data with the meta data that resided in the ID3v2 tag, i.e. ARTIST/ALBUM/etc...
The undo function doesn't recreate the MP3Gain information in the APEv2 tags apparently. So I'm proposing a MP3Gain safe-mode. When activated it will not overwrite the APEv2 tag or the MP3Gain specific tag fields no matter what Read/Write/Delete settings are set.
I have to change the Read/Write/Delete settings all the time, because my Pocket PC's software player only understands ID3v2.3 (latin1) correctly, for my MP3s on my PC I use ID3v2.4 and for my Wavpack files I use APEv2... especially when I'm in a haste and I want to prepare some MP3s for my Pocket PC, I tend click the wrong settings or save the tags while the settings where still set for a different type of files and then sometimes the MP3Gain values are gone for good... luckily I work on copies before I send them to the Pocket PC so I only have to re-copy or convert the albums and then re-mp3gain them, still it's very inconvenient UI-wise.
If that's not going to happen, can somebody tell me about a better alternative than MP3Tag when you have to prepare audio for different players quickly? I use foobar2000 for listening at home exclusively (APEv2 for WV/ID3v2.4 for MP3) and TCPMP on my Pocket PC (ID3v2.3 ISO 8859-1).