I don't even think it has to be something big to be honest. For all we know, it might be something absolutely weightless like Picard mentioning in passing he had met a new admiral character as a young Lieutenant on a mission to Cardassia a few weeks after the Dominion War ended, and some people will yell all over the internet in all caps how this is impossible as he was supposed to spend his 318-day shore leave on Ba'ku that he talked about at the end of Insurrection at that exact time, and how this proves once again that Kurtzman/CBS has no respect for teh canonz, it's ruined forever and it's #NotRealStarTrek.