The classic X-Men frenemy Magneto has a forgotten mutant power — the ability to astral project. Let's face it, the Master of Magnetism is not someone you want to underestimate. He's recently been confirmed one of Marvel's Omega mutants, defined as someone "whose dominant power is deemed to register — or reach — an undefinable upper limit of that power's specific classification."
Magneto's feats easily justify his classification as an Omega. At times he's bonded so closely with the Earth's magnetic field that he's almost dissipated into it, while in the classic "Fatal Attractions" story he unleashed an electromagnetic pulse that swept across the entire planet, causing all electrical equipment to malfunction. Airplanes fell from the sky, hospitals struggled to restore emergency power, and the X-Men scrambled to respond to Magneto's aggressive tactics. Now that all the mutants have gathered on the living island of Krakoa, Magneto has become one of Krakoa's greatest heroes, always on the front line in defending Krakoa from aggressors. But as powerful as he may be, he has a forgotten power that hails all the way back to the classic X-Men comics.
Magneto first demonstrated the power of astral projection back in X-Men #4, all the way back in 1963, when he took a psychic stroll and had an argument with Charles Xavier. "I will never tell you where I am," Xavier insisted, "but we can meet on a mental plane!" Two issues later, Magneto repeated the trick when he projected his mind out of his body in order to quest for the Sub-Mariner. Astoundingly, a text box inserted by Stan Lee himself clarified that Magneto's "brain power is second only to that of Professor X."
The issue appears to be that, back when Stan Lee was first creating mutants, he initially associated them with psychic abilities. Lee's first mutant story (published a year before the X-Men made their debut) featured a telepath and telekinetic, and it's worth noting he was least interested in the X-Men's Jean Grey — to whom he gave his default powers. Thus it's likely Lee simply considered all the most powerful mutants to possess psychic talents as well, explaining why he portrayed Magneto having out-of-body astral strolls.
The power was essentially forgotten for decades, until John Byrne's X-Men: The Hidden Years was published from 1999 onwards. He told a story in which Magneto used this power of astral projection to impress a superstitious tribe in the Savage Land, only to draw the attention of the X-Men despite his best efforts; Byrne also repeated the claim from X-Men #6 that Magneto's mind is second only to Xavier, and had him restrain Jean Grey's own telepathic powers. But Byrne appears to be the last person to remember this particular curious talent of Magneto's, and it's safe to assume it won't be touched on again.