"Was it X that meant you couldn't do Y?" is the best way to phrase it if possible. It demonstrates an attempt, even if it's just a simple one, to understand it, and implies you have faith the other party haven't missed something obvious. Thinking of an X often answers the question for you, stops you looking stupid.

Every now and then, the reply is "that would have been a good idea actually" in which case you still get to look smart. So it's a win+win really.