I have a MAD story, but nothing as cool as Jason's (working right above them; well, maybe it's close, you be the judge). I will attempt to provide the Reader's Digest version.
When I was in high school (during the Wacky OS run), one of my best friends and I were both heavily into MAD magazine and the paperback books. We weren't old enough to work, so to buy each issue [every 6 weeks or so], we had to save up our allowance. Issues then were $0.40, lol. We lived in northern NJ, so were fairly local to where the MAD offices were in New York City.
My father was also somewhat into MAD at the time (he actually had almost a complete magazine collection, including the first magazine issue from 1953, I believe) and at some point, he offered to take us into Manhattan to visit the offices and, for the main reason, to see if we could find some of my missing MAD paperbacks to fill in the holes in my collection. I think I had 70+ of them by that point, but you had to actually go to book/department stores to find them then and some were elusive.
The MAD offices were housed on MADison Ave in the city then. We got there and rode up the elevator to the right floor. Just to the right of the elevator exit was an impressive wooden door with a big MAD logo on it, so we knew we'd found it. We walked in and, to be honest, I don't recall a lot of what happened next, except that a receptionist greeted us nicely, we explained why we were there and she had us "come on in". We went thru another door, down a narrow hallway and at the end of said hallway, we looked thru an open office door on our right....and there, at his desk, was William M. Gaines, on the telephone! How cool!! We waved and my friend said "Hello, Mr. Gaines" and he waved back! Just neato, as we'd say in 1974.
We turned to our left and walked down another hallway and I remember at the end of same was where there were artists sitting at drawing boards, desks, etc. working in an open room. I wish I could tell you for certain which artists/writers were there, but for two reasons: it was about 45 years ago and I was so overwhelmed at being there, my brain got fuzzy. We were guided back down the hallway a bit to a room on the side which was filled with back issues just strewn about (which they let us take some) and, as I had hoped, MAD paperback books galore! Yay! I don't remember exactly what I bought/got that day (see above reasons), but I do recall being ecstatic at my haul and visiting such a gloriously MAD place.