I'm not sure if this could be considered a bug, but it really bugs me.
If I accidentaly select a space before or after the text which I'm copying to any of the tags, it will be saved. For example, if I copy an artist's name from the internet and accidentally select a space before it (very easy to do in a body of text), that space will become the first character of the artist field after saving it, and that artist will show up first when sorting artist names in a list in any audio manager. Also, it's very easy to overlook such things while viewing files in Mp3tag, and it seems to do this for any tag field.
I don't know if there is any special case in which having spaces at the beginning or end of tags would be justified, but if there isn't I would like to recommend changing Mp3tag's default behaviour to deleting those spaces when saving, similar to the way Windows automatically delete them when you try to rename a file.
Other than that, Mp3tag seems pretty much flawless.