Some of this usage might be documented in versions.txt but this discussion actually came up on IRC with Sat, he ran a code testing all identifiers using :error from a list I had, the list wasn't that complete (~20 missing maybe) but it should be good:

$+,$++,$?,$?*,$*,$cd,$feof,$ferr,$getdir,$inmidi,$inmp3,$insong,$insong.pause,$inwave,$logdir,$mididir,$mircdir,$mircexe,$mircini,$mouse.key,$mouse.x
$mouse.y,$mouse.win,$mouse.mx,$mouse.my,$mouse.lb,$mouse.dx,$mouse.dy,$mp3dir,$script,$scriptdir,$scriptline,$snicks,$wavedir


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