It can be tough to break the habit of calling people by one name, then switching later. I went to school with a Tommy from elementary on up. Didn't have much occasion to hang out with him, but always thought of him as Tommy. Freshman year of college, I came home for a break and brought one of my friends. Introduced her to Tommy when we ran into him around town, and cue major embarrassment from me when he gave me an awkward look, and said: "Actually, it's just Tom." Apparently he'd decided not to go by the diminutive at some point, and I'd missed the memo. Whoops. I still have to mentally adjust the name when I run into him now.
On the other hand, it was quite a shock to meet my college roommate's parents and realize that they called her a completely different name from what I did. Picture me asking: "Where did Elizabeth go?" and being answered with: "Bethy's in the kitchen." I'm standing there thinking, "Bethy? Who's Bethy?"