I think you are looking for an example where there might be a consequence for failure. I would suggest starting at the simplest example first. An opposed roll to knock someone prone (can be done out of combat). What is the consequence for failure in this case?
It could be a bunch of things. (I'll get back to that in a second.)
But I think one of the good points to come out of this thread is that it's not necessary to look for a the consequence of failure of a proposed action (or a roll, if you're in the camp that those two things are synonymous). That's getting things backwards. Rather the point is to ask: is there a consequence for failure? If so, this might be something to roll for.
But if not, if there's no obvious negative consequence of failure...maybe you don't need a dice roll? Take the medicine example: maybe it's really
not likely that you're going to make the guy sicker. And maybe...like in your (was it your?) narration, there's some herb that's been mentioned in the game...or maybe even the player just makes all that stuff up and that's ok too...why not just give the guy an autosuccess? I mean, he's got proficiency in Medicine, right? What does it add to the game to make him roll? I think that's the bigger lesson here. Bigger than the question I was asking in the thread, really.
Ok, back to your knocking prone example:
1. If it's in combat, then obviously you use your turn, and if you fail you've wasted it.
2. If it's
not in combat, I would think the most obvious consequence is that you're probably now in combat.