Side note #1: Its much easier to write on a Magic card with a marker rather than a roller ball pen.
Side note #2: Actually I used to carry the marker with me all the time, you never knew when you might need to proxy some cards for an emergency playtest session.
This situation happened in the summer of 2007, at some European Time Spiral block constructed Grand Prix where I played Blue/Black Teachings, can't remember exactly which event (I played Teachings in all Grand Prixs of that format). During my Byes, I was playing some games against Tomoharu Saito playing Mono Red (also doesn't help remembering that much since he probably played Mono Red in all those Grand Prixs), while some friends were watching.
At some point a player gets by and asks everyone for signatures on cards, playmat, etc. Everyone's happy to do it of course, I think no Pro Player would ever refuse to do it, except for occasionally Kai.
Side note #3: In the distant year of 2003 I wanted to get a Voidmage Prodigy signed by Kai, but I was afraid Kai would just say "No". My friend Rui Oliveira was at the time the Sideboard coverage reporter for European Grand Prixs, so I gave him the card and asked him to go find Kai in the Grand Prix to sign it, hoping that Rui as a sideboard reporter would have better chances to get the card signed, rather than me, a poor lonely no one.
Back to the awkward situation, the guy also asked me to sign a card, the problem was he handed me over this specific card to sign:
This card, Rakdos Augermage is the card Terry Soh created, oops sorry Terry, I mean Wizards created for Terry Soh for his Invitational win in 2005, and the guy pictured in the card is also supposed to be Terry Soh, as it is a long tradition to portrait the winner's face in the card he creates. So usually you ask that specific person to sign that specific card, just like I kind of did (or asked Rui to) with Voidmage Prodigy and Kai.
So here's what happened when I was given the Rakdos Augermage.
Me: You know, I'm not Terry Soh...
Him: You don't want to sign it?
Me: It's not that I don't want to sign it, but this specific card kind of represents Terry Soh
Him: No, it's ok, you don't want to sign, I get it. Thanks a lot man, sorry for disturbing your game.
Me: You're getting it wrong, I'll be happy to sign a card for you, but perhaps a different card would be more appropriate.
Him: You don't want, you don't want, it's ok, no need for excuses.
Me: I'm not making excuses it's just that I feel... ah, you know what, forget it, just give me the Rakdos Augermage.
Him: Whatever.
Me: Do you want me to sign my name, or just write "Terry Soh" on it?
Him: Meh, up to you, I don't care anymore.
I obv signed "Terry Soh". And that my friends, despite our similar looks, was the closest I've ever been to being Terry Soh. Judging from his facebook pictures, I wouldn't mind occasionally switching with him and living "The Fast and the Furious: Tokyo Drift" style of life.
To Terry: If you ever come across that Rakdos Augermage, sorry about that!
To Oli: Stop saying "Hi Terry" everytime you meet me at an Asian tournament.