New
#11
Another item I found is that WMP12 really, REALLY doesn't like ID3v2.4 tags. I also found that WMP11 will put it's own ID3v2.3 tags on top of any ID3v2.4 tags.
I found this by running mp3tag on my collection. All my music was tagged with v2.4 going in, so I was able to quickly find the files that WMP11 had messed with. If the tag was identified as 2.3, I cleared the tag. The 2.4 tag that was originally there was still there.
Then after all the 2.3 tags covering up 2.4 tags were removed, I was able to run Musicbrainz Picard on the collection again, telling it to use 2.3 tags instead. This time all the music (more or less) was found. Any remaining unknowns were quickly rounded up and fixed.
BUT... What I ALSO found was that WMP12 will mangle the Musicbrainz hash tags if the Retrieve Additional Information from the Internet option is turned on.