Originally Posted by Dark Raven
Maybe they'll say that continuity has already been messed up because someone has already time travelled within the films without them being aware of it. So rather than seeing a future traveller going back and then potentially disrupting the timeline, maybe someone already has done that all this time but we just don't know it yet. Perhaps Cable or whoever will look at all the events of the films on a screen and wonder why nothing seems to match up.
It could work if, before they time travelled in the future, the X-Men (and whoever was with them) became aware that several timelines existed (either a machine detected the timelines, or a mutant like Destiny sensed them), and there was the risk that whoever travelled back would end up in the wrong timeline.
Bingo! I've sorted it!