Encounters that didn't work

[RexDart]

A good way to deal with this is to just say "You lose, your captured whatever, maybe narrate it a little". Then give everyone some sort of boon/action point to make up for it.

I think "caught in an inescapable trap" is somewhat more satisfying in a D&D narrative than "unwinnable fight" if you must have this sort of setup. Especially because the characters likely will fight and defeat the "unbeatable" foes at some point.

We're ultimately talking about a form of railroading, which is usually a necessary evil in any D&D campaign to some extent. The key is getting buy-in from the players that the railroad trip will be a fun one, so they're not constantly scheming to derail the train.