We just encountered a file that liquidsoap failed to decode (for my record, "/home/savourey/musique/Portishead/Roseland NYC Live/ 01 Humming.mp3") and it took me some expertise to figure out why; I'm afraid a normal user would have been lost.
The issue was that the MP3 file starts with stereo but has a mono frame somewhere near the beginning, so that its decoding gets interrupted. First, the only informative error message is in the global log, not in the request log. Second, the message is not so informative (says "mono instead of stereo") unless we set up a mono_to_stereo decoder in which case it becomes "non-uniform content type". Last, should we try to allow that kind of quirk (I think there's a reason to avoid it but can't remember) and should we try to fix it silently for the simple and common case of MP3? The MP3 file is played OK on most players, as expected.