Doctor Who has a fair number of these, with the Outpost Gallifrey "Gallifrey One" conventions being fairly well known. Peter Davison turned up to a few in his Fifth Doctor costume, before deciding that since others were doing it, it was a bit odd and stopping. Of course, Patrick Troughton famously died at such a convention.
Because it's the only example that's about a series actually having conventions, rather than conventions appearing in the series.
Pulled this:
Because it's the only example that's about a series actually having conventions, rather than conventions appearing in the series.