Recently on the ML, a user had

output.icecast( name = myName, host = myHost, genre = myGenre, description = myDesc, url = myUrl,
               password = "***", port = 4040, mount = "high.ogg", restart = true,
               %vorbis(quality = 0.6, samplerate = 44100, channels = 2),
               return )

which resulted in

2011/04/15 12:23:50 [threads:1] Thread "wallclock_main" aborts with exception Vorbis.Not_implemented!
2011/04/15 12:23:50 [threads:3] Raised by primitive operation at file "ogg_formats/vorbis_encoder.ml", line 98, characters 4-57
2011/04/15 12:23:50 [threads:3] Called from file "ogg_formats/vorbis_encoder.ml", line 158, characters 19-96
2011/04/15 12:23:50 [threads:3] Called from file "encoder/ogg_encoder.ml", line 213, characters 25-48
2011/04/15 12:23:50 [threads:3] Called from file "encoder/ogg_encoder.ml", line 213, characters 25-48
2011/04/15 12:23:50 [threads:3] Called from file "list.ml", line 69, characters 12-15
2011/04/15 12:23:50 [threads:3] Called from file "list.ml", line 69, characters 12-15
2011/04/15 12:23:50 [threads:3] Called from file "encoder/ogg_encoder.ml", line 215, characters 6-24
2011/04/15 12:23:50 [threads:3] Called from file "encoder/ogg_encoder.ml", line 215, characters 6-24
2011/04/15 12:23:50 [threads:3] Called from file "outputs/output.ml", line 247, characters 24-77
2011/04/15 12:23:50 [threads:3] Called from file "outputs/output.ml", line 258, characters 35-47
2011/04/15 12:23:50 [threads:3] Called from file "outputs/output.ml", line 174, characters 6-27
2011/04/15 12:23:50 [threads:3] Called from file "list.ml", line 69, characters 12-15
2011/04/15 12:23:50 [threads:3] Called from file "clock.ml", line 131, characters 6-43
2011/04/15 12:23:50 [threads:3] Called from file "clock.ml", line 307, characters 10-24
2011/04/15 12:23:50 [threads:3] Called from file "clock.ml", line 310, characters 8-15
2011/04/15 12:23:50 [threads:3] Called from file "tools/tutils.ml", line 136, characters 15-18

The explanation from Romain was that quality = -0.2 is only available in the aoTuv implementation of libvorbis, therefore you have the exception "Not_implemented" which probably means that you are using xiph's libvorbis..

We should check for the range of parameters, or at least catch the Not_implemented exception and display a meaningful error message.