BTW, if you're perhaps unaware of what "contradictions" i'm talking about, here are a few examples taken from the timeline page here on the Wiki, along with some i've added myself.
InX-Men, Charles tells Logan that he met Erik when he was 17 years old. In X-Men: First Class, they met each other in their early 30's.
In X-Men, Charles doesn't know why he can't find Erik with Cerebro, whilst in X-Men: First Class he witnesses the effects of the helmet first hand.
In X2: X-Men United, a human Hank McCoy can be seen on a television screen debating the issue of mutant rights alongside a "Dr. Shaw". In X-Men: The Last Stand, Kelsey Grammer portrays Beast in full blue costume. (There was a scene written in the screenplay for X2 that showed McCoy undergoing his Beast transformation in the scene where Xavier activates Stryker's Cerebro to target all the mutants, but it was cut early on.)
In X-Men: The Last Stand, Moira MacTaggert is a Scottish doctor working at Muir Island's genetic facility, while in X-Men: First Class, she is an American agent working for the CIA.
In X-Men Origins: Wolverine, as well as the opening scene of X-Men: The Last Stand, Charles Xavier is shown to still be friends with Erik Lehnsherr, and still have the use of his legs, whilst in X-Men: First Class he is (accidentally) crippled by Erik, and the two part ways to form the X-Men and the Mutant Brotherhood respectively.
In X-Men and X2: X-Men United Charles and Erik say that they built Cerebro together, whilst in X-Men: First Class Hank McCoy builds it himself, and shows it to Charles and Erik.
In X-Men and X2: X-Men United it is said that the Weapon X program took place 15 years prior (which would be around the mid-to-late 1980's) while in X-Men Origins: Wolverine the program takes place in 1979.