When it all comes together...
Two weeks ago, my players encountered Duergar in a small dungeon. After a Medium and a Hard encounter back to back, they were trapped by a cave-in. I told them they only had enough air to take a short rest before they would have to unbury themselves, but they came up with an ingenious idea and so were able to take a long rest.
I considered the design of the Duergar sector. It had been set up with roving patrols (random encounter) and there were several more major encounters ahead. But now that the Duergar had the party trapped I realized this could be tough. I warned the players about this, but, well, I'll continue.
So they extricate themselves from the cave-in. They are rejuvinated, having fully rested, and with a brief social encounter they recruited three NPC prisoners to join their cause. They find that the Duergar had sealed off the immediate escape route to funnel them towards what I hoped would be a challenging encounter with either a short rest before the next one, or a chance to negotiate with the leader to avoid it entirely.
That's not what happened. The party had a rough time with the CR 1 Duergar I'd put in their path, supported by a caster. During the fight, one player tried to escape, where they blundered right into the second encounter. Seeing a lone, wounded PC, I decided to roll with it and so the second encounter started immediately.
They won, but it was a very close thing. The multi-stage battle lasted most of the game session (4.5 hours). Everyone was exhausted afterwards (IRL and in game), so I awarded a ton of xp for their troubles, and creative solutions.
... You're framing this as problem, but with just a positive outlook and a couple minor adjective changes, it sounds like a really fun session. It had outside-the-box thinking to deal with the cave in, a social encounter with the freed slaves that turned into a combat boon, and by-the-seat-of-your-pants action. I'd be happy playing in this game.