The year for the Showtime series Continuum has gone (tt1954347) - possibly this is something to do with the other 2012 series Continuum (tt2309131) being added?
Hmm - now that I look (all movies, ordered by year), there's about 250 with ???? as the year (or start year for series) - probably easier for you to see yourself than try to list all of them here
- it looks like some of them are 'in development' (so no year on IMDB); most of them are individual series episodes that have sneaked onto the database; or ones like Continuum above, where there are more than 1 series with the same name in the same year, e.g. Millenium (tt0115270), Pawn (tt1551630), maybe adding the second one corrupts the year for the first one?