The "card on forehead", as done by Michael Finney and probably by other magician, is for my opinion hilarious. I still remember how much I laughed when seeing it for the first time. However the last time I performed it, I had some thoughts about it. I did it in front of a group which was familiar with one another. After finishing the group continued to mark the guy that was with me. I felt uncomfortable on putting him in such a position. I guess any sucker effect is a little uncomfortable for the spectator, but this trick makes a sucker only of one guy and not from all the audience.
I will appreciate your thoughts on this subject.

What it means to "mark the guy?"

I was wondering the same thing.

I think he means "mock."

I think any routine that mocks or makes fun of a spectator, except in special circumstances (such as among a bunch of performers) is a bad idea. The sting can be be lessened by ensuring that the "victim" understands it is all in fun, but it still probably hurts somewhat. For others to continue to "rub it in" is inexcusable. I enjoy Michael Finney's performances immensely, and I would like to believe that the "victim" is in on the gag or otherwise prepared beforehand.

No the victim is NOT in on the gag. The point is to make them feel less like an idiot, which is tough to do.

Blame my bad english. I meant ofcourse "make fun of the guy".

So then, how can one lessen the sting? Any suggestions?

As funny as this routine is, I don't do it for that reason. Even when Michael Finney (arguably one of the best at it) did it on his DVD series, I felt bad for the spectator and though Michael made him look like an idiot. The spec laughed and looked to be at least pretending to enjoy it, but I don't think he really did. I know some people can pull this (and other sucker effects) off well, but it definitely takes a very likeable personality, and even then there's a risk of offending someone, or if not offending them, at least embarrassing them and turning one more person away from magic and magicians. Handle with care!

The above post should say "THOUGHT Michael made him look like an idiot," not "THOUGH Michael made him look like an idiot."