Originally Posted by Fourputt
While I agree with the sentiment to call the guy on it, match play is a different format and Tomvk77 has the best answer in this case. Inform the player with the extra club of his breach and see what he does. You are neither referee nor committee, so calling it yourself is just going to cause a lot of bad feelings. If the guy has any self respect at all he is going to go straight to his opponent and explain, take the penalty of the loss of 2 holes, and accept the consequences.
I agree with the idea that as an outside agent (not referee, nor committee, nor player) that it's not something I'd really want to get involved in beyond informing the offender. After that. you get to learn about both of them based on how each of them handles it from here on out.
Had I been the buddy with the 15 clubs, I cannot imagine that I wouldn't have noticed it myself while showing my new clubs to my friend, but assuming I didn't and then he did inform me, I absolutely would have gone to my opponent and told him of the problem and taken my penalty.
But here's where it gets messy for me. Had I been the opponent, and this guy comes to me and explains that he just got new clubs, accidentally left his old wedge in there even though he never used it nor intended to, and thus deserves a penalty, I would say don't worry about it, and let's just finish the match. I know that's against the rules as well, and if the other guy was really adamant about it, I'd go ahead and accept it, but assuming he's just a normal, laid back guy (he's my friend, so that's probably the case ;)) then he's not getting a penalty. We're playing the last 2 holes with me 1-up, and that's that. I would not want to win a match by what basically amounts to a forfeit.