At the X-Mansion, Deadpool is wearing Professor X’s signature headset, and when the actor — Ryan Reynolds — attempts to mess with it, a piece of it breaks off.
The scene in question occurs in Deadpool 2 right before Wade, Colossus (Stefan Kapicic) and Negasonic Teenage Warhead (Brianna Hildebrand) head out to help Firefist (Julian Dennison) in his introduction scene.
Believing it to be funny, David Leitch confirms on the DVD Commentary Track that they just left the moment in, telling Reynolds later that it was a real archival prop, and probably of some value to sentimental X-Men movie fans, everywhere.