Last night, my players decided to head off looking for the cave they were told about. For some reason, they ignored the NPCs trying to help them and didn't want directions.
I let them fail.
Eventually they went back to town and asked for directions. At that point, I did a fail forward by asking for a check to see how fast they were able to follow the directions. If I hadn't let them make just the one check, they wouldn't have gotten anything done on that bounty.
This, to me, is a perfectly viable example on how to use fail forward. If you think this was wrong in my judgement, please let me know how you feel that way.