For example - Say the PCs are exploring a tomb, hunting the BBEG, who is in his secret lair, behind a super-secret door. The players go through the dungeon, search for for secret doors, but they botch the roll, and fail to find it.
This actually sparks an example from a game several years back. I'd consider it acceptable, in terms of "failing forward", but would be interested in what others thought.
The PCs needed to retrieve some widget from a dead sage/artificer's lair. I don't remember what it was, but let's say a sword. The sword was well guarded (think Tomb of Horrors lite), but quite within the PCs ability to reach. They were under a time constraint and dangerous things occasionally spawned. They managed to get the sword but discovered that the sage also had a journal that revealed things about the campaign's BBEG and said journal was still on premises. They tried, but could not find it in a reasonable amount of time -- need to use sword to stop bad thing.
A couple months thereafter, the Warlock went back to the lair to search. Why did he go alone? 1) He's a CN Warlock. 2) He was uniquely equipped to deal with some of the dangers, so long as other PCs weren't standing in the danger zone.
Based on #2, all he needs is time and a bit of OCD. Eventually, he finds the secret door in the ceiling that led to the secret lab, gets the journal, and the group has an advantage in the next act.
I think that's within bounds because 1) the journal wasn't important to the PCs' immediate goals, 2) it made their long-term goals easier, but wasn't a "blocker" to the campaign, and 3) it was obvious to at least one player how to work around it and what the cost was. Even #3 wasn't critical to the "fail forward" concept.