I have discovered something strange; I don't know whether or not it's a bug, but it appears to be. I used mp3Test to check my music files, and it found errors in one folder, from .44 to 1.15%. These files I originally got from vinyl, using Sound Forge, to produce wav files, which I then used dBPowerAmp to convert to mp3 (dB gives better quality options than SoundForge.) I never needed tags, so I did not add tags until about 4 years ago.
I just found an old external hard drive I had misplaced for several years, and it had my original conversions of those files, the mp3s I had made immediately after converting the vinyl to wav. These files had no tags.
These original files all had very low "error" rates, typically .02% or so. So I copied the tags from the files with errors, using MP3Tag, and pasted the tags from the files with good tags but with errors, onto copies of the old, low-error files. These files with fresh tags now all had high error rates, identical to the ones in my main archive. So I tried again, this time doing the conversion manually--via the ID tag editor, one line at a time, and manually copying the album art, using MP3Tag to put it on the Windows clipboard, and pasting the album art via the properties tab. The manual conversion had higher error rates than the original, but not as high as my archived.
For instance, for the file "The Surrealist Waltz," the original file with no tags showed .02% error. The file on my "main" storage (the one I keep to listen to the music from) showed .63% error. I copied the clean file, the one without a tag, and put the tag on two different ways, with mp3Tag, and manually. Manually, I ended up with a .27% error, but with mp3Tag, I ended up with .63% error, the same as my "main" archived file. (The "manual" way involves dB poweramp's integration into my system, not pure Windows.)
I have just sent a query to the maker of mp3Test to see if they know of an explanation, but I thought I should post this here also. Since I use non-standard tags sometimes--for instance, there is a tag, "Comment," with the words "from vinyl" on each of these tags--but that doesn't explain why there is a different error rate doing the same thing manually and with mpTag.