I decided to consult some experts to end this debate once and for all. I contacted two sources. The first was a "Grammar Hotline" that is run by the University of Maryland English Department. Their phone number is (301) 405-3785. I explained the bet and provided them the exact wording as proposed by Vegas. The woman I spoke to felt very strongly that Jsearless is wrong. She maintained as I did earlier that for his interpretation to be correct the sentence needed to have read: How about I find, let's say 15 posts, where you have used each of those two words incorrectly? She also argued that misuse of one of the words constitutes misuse of both as it demonstrates a lack of understanding either word. So Vegas wins correct?

Well then I sent an e-mail to Susan B. Kline a Business Communications expert with a BA and MA from Harvard. Ms. Kline also runs a "Grammar Hotline" but via e-mail. Here is her response:

No matter what the intent of the bet, the wording was indeed loose. The loser is correct: You had to find 25 posts in which both “then” and “than” (“those two words”) were used incorrectly. The wager was more likely intended to be: Find 25 posts in which either “then” or “than” was used incorrectly. ..but that was not what the wager stated.

The loser has grounds for refusing to pay. If the bet were not so large, I’d say, “Be a sport,” – but not at $500!
So she sides with Jsearless but does seem to indicate that he is an angle shooter. In light of differing opinions from expert sources, I think a ban is not appropriate but Jsearless absolutely should be stuck with an angle shooting piece of shit status.