Imaro
Legend
I see "fail-forward" (or D&D 5e's "progress combined with a setback" or "success at a cost") to be just another form of stake-setting. On a successful check, you succeed at your goal. On a failed check, you succeed at your goal with a cost or complication. I use it when outright failure wouldn't be particularly interesting or when the a binary pass/fail would result in a disconnect between player and character knowledge.
For example, many people suggest rolling ability checks secretly when players try to search for traps, knowing that if a player sees a low result on the die and hears the DM saying "There are no traps," "You believe there are no traps," or "You find no traps," the player may be tempted to repeat the task or have another character make a pass at it. (Cue the cries of "Filthy metagamer!" Not that I give even a single flumph when players "metagame.") By using progress combined with a setback, I can narrate a failed check as being, for instance, "You find the trap - and your foot is on the pressure plate! There is a continuous, disconcerting clicking noise coming from the walls around you. What do you do?!" I thus avoid that disconnect and don't have to take the dice from the players as others DMs do.
So question... in a situation like the above... when has the trap actually been sprung (when do I suffer it's effects??). It seems like just in looking for the trap and failing you've now put my character into a situation where he's (partially??) sprung the trap just by looking for it. Not sure this would be cool with me as a rogue since it would mean when I fail at searching for a trap (not necessarily doing anything to spring it) I then end up guaranteed to suffer it's ill effects... if I fail to disarm it... well, and since I'm the rogue I'll be searching (and failing) at a higher rate than other characters.