First, thanks for an awesome program.
I was unable to find any information about what Mp3tag does if both id3v1 and id3v2 tags exist but the tags contain conflicting information, so I did a few experiments and the results weren't that great. What I found was that id3v2 always overrides id3v1. This is true even if the id3v1 fields are populated while the id3v2 tags are not.
So, when opening an mp3 where both tags exist but v2 is not populated, the mp3 will seem as if it does not have any tags. All the id3v1 information is hidden.
When saving the mp3, all the id3v1 tags will then be silently overwritten with the (possibly blank) id3v2 tags.
This is the behavior in the default configuration where all tags are read and written. Turning off read or write for a specific tag does not really solve this problem.
What I would like to see is some kind of warning when tags are being hidden and when hidden, populated tags are about to be overwritten with different or blank data.
Also, it would be great if Mp3tag would automatically display (and make available for editing) both id3v1 and id3v2 fields if they are different, or at least visually indicate when a field displays a value that is not consistent between tags, for instance by graying out the background in the field.