I'll admit to being an ignorant Yank who has only rarely attended a proper session, and who (like any good 'Murican) gets much of his education from the movies. That said, I recall a scene in "The Boys and Girl From County Clair" where the bandleader berates the young whistle player (who had thrown in some improvisation to make it more "interesting") for "playing AROUND the tune, not PLAYING the tune". Granted, this was supposedly in rehearsal for a competition between ceilidh bands and so might be a different paradigm from an open session.
Assuming that scene reflected SOME real-life attitudes, that would imply that there are some people who believe there is only one "right" way to play a particular tune.
Personally, my playing is so sloppy that I can never do something the same way twice, so I just tell people it is improvisation and act like I MEANT to play it that way rather than admitting to a mistake. [grin]
And the advice I gave my sister, who is starting to occasionally attend sessions in Oklahoma: "When in doubt, trill"