A command such as "JOIN #a,#b,c" will join the channels "#a" and "#b". However "c" is not a valid channel name and is ignored. I would expect to receive an error message if I try to join an invalid channel. Libera.Chat uses ERR_NOSUCHCHANNEL (403) in this case.
A JOIN command with channels and keys can result in mismatches between the two:
JOIN #foo,#bar,#baz foo-key,@bad-key,baz-key would result in these channel-key pairs: (#foo, foo-key), (#bar, baz-key) and (#baz, ).