Windows 10/ Audacity 2.4.2:
When I try to import a particular MP3 into Audacity, I get a “MP3 Decoding Failed: CRC check failed” error. The MP3 file plays fine in iTunes and checks out fine with MP3 Diags.
One thing I noted in MP3 Diags is a “CRC=Yes” in the stream details, where every other file I’ve looked at has that set to “No”.
The problem with Opus import was because Audacity was too permissive with non-standard MP3 files, so Audacity was getting confused between MP3 files and Opus files. That was fixed by making Audacity more strict about MP3 files. Unfortunately it has since been found that Audacity is so strict that some MP3 files will be rejected even though the “errors” in the file are quite minor (such as the presence of APE tags in the file).
The next version of Audacity is a bit more forgiving about non-standard MP3s.
In the meantime, you can probably work around the issue by using a different app to convert the MP3 to WAV (for example, Foobar2000)