I think there were also some contradictions with the first two films. Off the top of my head, I think there was a bit where Magneto mentioned helping Xavier to build cerebro, which clearly wasn't the case in 'First Class'.
Sure you could reason that in the several decades between FC & X1 Chuck and Erik may have gone through periods of further collaboration, but if we're just going off what the film tells us, that's not implicit. Now Nightcrawler's parentage on the other hand: that's fairly implicit.
