Though the decision to tie together the two X-Men franchises gives fans the mouth-watering opportunity to see two stellar casts together at once, as well as an iconic X-Men story arc, there has had to be some trade-off. There is no way X-Men: First Class was supposed to tie-in to the first X-Men timeline - despite the cameos - and ret-conning in order to mesh a reboot with the thing it intended to reboot was inevitably going to be problematic. The cost, somewhat inevitably, is in continuity. And not only that, as with pretty much every time travel movie - including the very smartest - continuity errors will happen, which isn't at all surprising considering the fact that continuity editors can fail to spot transitions between scenes that run concurrently in normal movies, let alone sticking to details in twisty timelines. And therein lies the rub: despite the mounting Sentinel threat, and the looming possibility of all of human existence being wiped out by an over-zealous cleansing programme gone wrong, it is neither that represents the biggest threat to the X-Men universe: instead it is continuity who is the biggest foe. Because Bryan Singer and Fox have decided to unite the time-lines, they've basically invited any X-Men fan with even half an interest in continuity (which should be EVERYONE) to pick faults. Fundamentally, we're all being asked to forget some pretty key continuity elements that have been, in one way or another, contradicted by the rebooting of the time-line, like having two completely different Sabre-tooths in the same universe and not having his brother notice the difference, and those are far from the only things Fox would prefer we didn't remember...
