Wolverine agreed to take care of a young Japanese girl named Amiko at the request of her dying mother. The girl is presently being watched over by Logan'y ally, Yukio. (Uncanny X-Men #181)
When Xavier left Earth with the Shi'ar to heal injuries inflected on him by an anti-mutant mob, he asked Magneto to run the school for him. Magneto did so under the name, Michael Xavier, but eventually left, unable to convince the New Mutants to follow his points-of-view. (Uncanny X-Men #200, New Mutants #75)
Before she joined the X-Men, Rogue was a member of Mystiques New Brotherhood of Evil Mutants. She came to Professor X seeking his aid with controlling her powers. (Avengers Annual #10, Uncanny X-Men #158, #171)
After their battle with the Adversary, the X-Men were saved by the celestial being called Roma, who gave the X-Men the ability to not be detected by any mechanical or magical devices. Since then, the ability has faded away. (Uncanny X-Men #227)
Cannonball once apparently died and returned to life, and was therefore assumed to be an "External," a rare mutant who is able to live forever. (X-Force #7)
Some X-Men aren't mutants. Mimic gained his power to duplicate others' powers after accidentally breathing in a strange gas in his father's lab, and Longshot is a genetically engineered humanoid from another dimension. (Uncanny X-Men #19, Longshot #1-#6)
Xavier once had a shapeshifter named Changeling pretend to be him while the Professor secretly prepared for an alien invasion by the Z'Nox. When the "fake" Xavier died in battle, the X-Men even held a funeral. The genuine article returned a short time later. (Uncanny X-Men #42)