I put "celebrity" in quotes because what I really mean is "person in the public eye who you are a fan of", regardless of how many other people would know this person's name/face/work. Best/worst can mean "they were awesome/they were a dickhead" or "I handled it well/I made a complete ass of myself" or "it was really cool/it was fuckin awkward". Living in NYC for almost ten years, I have a good amount of both types of stories...
I'll start with the better ones (although one of them is a mix of good and bad), and funny enough both of them involve me meeting the person twice.
I used to get tattooed at the same place as a shitload of heavy metal musicians, one of whom was Kerry King from Slayer. I wasn't getting work done by the same person, there was another artist there who I "clicked" with a lot better, and I still go to him even though he's not at that shop anymore. The first time I ran into Kerry, I was just popping in to pick up more of my guy's business cards because whenever somebody would ask where I got my work done, I liked to just give them his card. I walk into the shop, and Kerry's standing right in front of where all the cards are. He's instantly recognizable because he's bald and has tattoos all over his head, so I knew there wasn't any way to do what I wanted without waiting around like a weirdo or interacting with him somehow. I walk up behind him, calmly say "'scuse me", get the cards, and on my way out very nonchalantly say "oh hey, I like your work". We shake hands, and I made it about a block away from the shop before calling my cousin (who is also a huuuuge Slayer fan) going "Dude you'll never guess what the fuck just happened!" A few years later, on June 6, 2006, I was in the shop to talk to my artist about a design we were working on, and Kerry was there to get a small 666 tattoo to commemorate the date. I had my meeting with my guy in the back office, which happened to be where Kerry was hanging out with whoever else was around at the time, and since I was a shop regular nobody told me to fuck off so I just hung out and listened to everyone talk for a while. Finally after about a half hour or so I realize I have shit to do that night so I start saying/waving goodbye, when Kerry notices I have a B.C. Rich (the brand of guitar he uses) bass tattooed along the back of my forearm. I don't have it because of him or even because I play one myself, I have it because I play bass and that one fits the shape of that part of my arm really well. He goes "hang on a sec, let me see that again", I took a second to realize what he was talking about, showed it to him, he liked it, and I left knowing immediately it's a story I'll be telling 8 years later and beyond.
Stand-up comics are some of the easiest famous people to meet because at some point, they'll almost always end up hanging out in front of the venue for at least a few minutes at some point. I went to see a show that involved a few comedians I had varying levels of excitement for, second or third highest of which was Greg Giraldo. His set wasn't terrible, it just wasn't up to his usual level. Everyone has off nights, boo fuckin hoo. This is where comedy clubs' two drink minimum kinda bit me in the ass, because it was a long show and once I got started drinkin, I kinda lost count of what I had by the end of the night. Long story short, by the time the show let out I was shiiiiitfaced, and I ran into Greg. For some reason, it seemed like a grand idea to walk up and tell him "I've seen you do a lot better, and that tattoo bugs the shit out of me", so I did. He just said "ok" and walked away, and I immediately felt like absolute shit for saying it, but he was gone before I could walk it back. A few years later I went to another show he was gonna be doing, and again I saw him hanging out outside. I walked up and said something along the lines of "hey man, hopefully you don't remember me, but I ran into you a few years ago almost exactly like this and said some really fucked up shit, and I just wanna say I'm really sorry." He asked what I had said, so I told him, and I don't think he really remembered the specific incident but he just laughed it off and said "don't worry about it, it's all good", we shook hands and I fucked off to wherever I was going after the show. Three months or so later he died, and while I was really bummed about him being gone, I was glad I got to make things "right" before it happened.
This one isn't good or bad, it's just awkward and funny in a way that fits really well with this guy's work. My wife and I (back before we got married but were already living together) were on our way home from a night out, she was about 2 steps down the stairway to the subway station and I was about to go down, when I noticed Jim Gaffigan about 20 feet away walking down the sidewalk. We're both big fans of his so I said to her (quietly, I thought) "holy shit, look it's Jim Gaffigan!" I didn't think he could have possibly heard me, but he did so we all just spent a second or two looking at each other like "uuuuuuuuhhhhhh...", my wife and I awkwardly waved at him because we couldn't think of anything else to do, he waved back, and we all went about our night.
I was in the West 4th Street subway station on my way home from work when, walking along the A train platform, I noticed what I at first thought was some dickhead trying to copy Phil Anselmo's tattoos. About 2 seconds later, as he was getting on the train, I realized that it actually was Phil, and he was with Jimmy Bower (guitar player for EyeHateGod and Superjoint Ritual, drummer for Down)! By the time my brain registered all this information, they were in the train but we could all see each other through the window. I was so dumbstruck that the only thing I could think to possibly do was try to point out that Phil and I went to the same tattoo shop by pointing to a piece of work on my arm that neither one of them could have possibly known the meaning of while mouthing the name of the shop. To their credit, neither one of them looked at me like the idiot I was being at the time, they just waved and gave metal horns till the train pulled away. As soon as it was gone the rational part of my brain came back and was just like "why the fuck would you do that?!" To this day I still don't really know...
This last one I like to think I handled it as well as possible, but it was still weird... I ran into Joe Quesada (Editor-In-Chief of Marvel Comics at the time) in the bathroom at a New York Comic Con pre-party. Now, every rational person knows you don't try to interact with people in the bathroom, but this was probably less than a year or so after the whole One More Day debacle and I'm sure the poor guy had nerds accosting him in every possible situation spouting some variation of "U ruin spiydamang, u suxxorz!", so I can't fault him for what happened. I was washing my hands after doing my business, when I noticed somebody out of the corner of my eye, so of course I looked over. He saw the look on my face when I registered that it was him so he just did a quick wash and hauled ass out of there without drying his hands. I felt like a dick for unintentionally making him feel uncomfortable in the bathroom, so I finished up real quick, grabbed an extra paper towel from the dispenser and ran out after him. Once I saw him I started calling his name, he turned around looking resigned to another awkward confrontation, and I handed him the extra paper towel and slinked off without another word.
So that's my best and worst stories, what are yours?