I can post a bounty if I want to reward an existing answer. But what if it was question itself what helped me? I recently stumbled upon a question that alerted me to a problem I wasn't aware of. Quite possibly it saved me hours of pointless work. I can't reward it with a bounty as it would go to the answer, not the question (and it was me who answered it anyway).
Is a single upvote (and maybe clicking "favourite") the only way I can reward the OP's good eye for potential problems?
Note that this is not a feature request or anything. I just want to know, not to change the way things are.