Upon closer inspection, this seems to be related to bug #984, ID3 tags in FLAC files. Easytag has an option to add ID3 tags to FLAC files, which apparently was turned on for some time a few years back on whatever machine I encoded those FLAC files on.
http://bugs-meta.atheme-project.org/view.php?id=984 The question is if ID3 tags in FLAC files are valid – I've never had any problems with other players; and if Audacious should act the way it does, or either give a better error desciption or else simply play the files anyway.
