For example,
http://musicbrainz.org/ws/2/collection/142ea0d7-7fdf-4ea5-9b04-219f68023d01/releases
is a valid collection id, however
http://musicbrainz.org/ws/2/collection/142ea0d7-7fdf-4ea5-9b04-219f68023d02/releases
results in

Error:

Caught exception in MusicBrainz::Server::Controller::WS::2::Collection->releases_get "Can't call method "public" on an undefined value at lib/MusicBrainz/Server/Controller/WS/2/Collection.pm line 46."
Stack trace:
line 46 MusicBrainz::Server::Controller::WS::2::Collection
line 330 MusicBrainz::Server
line 304 MusicBrainz::Server
line 331 MusicBrainz::Server
line 345 MusicBrainz::Server
line 390 MusicBrainz::Server
Request data:

$VAR1 = {
'query_parameters' => {},
'body_parameters' => {}
};