Since X-Men: First Class considered a prequel in the X-Men film series, this was bound to happen.
- Xavier flirts with a girl who has heterochromia (where each eye is a different colour). In X2: X-Men United, Jason Stryker also has heterochromia.
- Xavier's pick-up line, "[Mutation] has enabled us to evolve from a single-celled organism into the dominant species on the planet," comes directly from the voiceover at the beginning of the first X-men movie.
- William Stryker is mentioned.Agent Stryker: I've seen this before at a magic show. Are you going to ask us to think of a number between one and ten now?Charles: No, Agent Stryker, although I could ask you about your son, William, who you were thinking about, which is very nice.
- Charles doesn't know it yet, but he'll one day become Bald of Awesome.
- There's a scene where Xavier uses the Cerebro prototype for the first time, and Hank struggles with fitting the electrodes on his head.Hank: Are you sure we can't shave your head?
Charles: Don't touch my hair.
- And near the end of the film, he jokes:Charles: I suppose I am a real professor now, aren't I? Next thing you know, I'll be going bald.
- There's a scene where Xavier uses the Cerebro prototype for the first time, and Hank struggles with fitting the electrodes on his head.
- Magneto rejects Mystique's human disguises and tells her she looks perfect in her mutant form. In X-Men: The Last Stand, when she is hit with the cure and reverts to her human form, Magneto abandons her, saying "She used to be so beautiful."
- When Mystique offers herself to Magneto, he utters "Maybe in a few years." She promptly turns into Rebecca Romijn, the older Mystique.
- When Hank McCoy finally finishes work on his appearance-changing serum, he calls it "the cure."
- Xavier's "For us, anonymity will be the first line of defense" sounds a lot like his line "Anonymity is a mutant's first chance against the world's hostility" from X-Men.