TANSTAAFL: There Ain't No Such Thing As A Free Lunch

That's still a better justifcation of your behaviour than the MIT students [edit: Yvain did not actually say MIT!] used - not to mention that you're able to look back in retrospect and acknowledge the error of your decision.

This sort of suspicion is a good heuristic, if not the best heuristic. Scam artists (by which I mean casinos and carnies) are skilled at making things appear as if you've found the loophole in their game, and when you don't have enough time to examine the loophole thoroughly you're generally better off assuming it to be false. From the sound of things you were too busy to do this, not to mention that - being unfamiliar with multiple choice tests in general - it caught you with your pants down. You would have had to devote twice the analysis time as a typical North American, who is familiar with these sorts of exams.

Don't discount the TANSTAAFL heuristic - you made a rational choice based on limited data and available processing time. Your error is wholly different from the errors at MIT.