^This.
While I was in residency, I did an out rotation and impressed a very prominent anesthesiologist, and he offered to write me a letter if I ever needed it. Three years later while applying for my current position, I took him up on his offer. At my interview, the first thing the department chair said was, "I know him from doing residency in that city. How do you know him?"
Later that same day, I learned that one of the faculty members had been the residency program director for my program director. He trained the guy who trained me. And he and two other faculty members did residency in the same residency program as me.
Medicine is a smaller world than you think. Never, ever burn bridges.