To be clear, I want to specify that I'm talking small twists here. We're not sweeping the rug out from the players so much as we're giving it a sharp tug to put them off balance, but otherwise leaving them be.
For example, if the players show up expecting to be going on a stealth mission through a castle, perhaps when they show up they get invited in to a party. It's still not a combat encounter. Their stealth stuff can still help. However, the encounter now has social aspects, interactions, and repercussions that could impact future games.
With combat, the players could show up expecting a mass combat only to end up in something much more personal and small in scale that they have greater direct impact over but is no less meaningful to the overall effort. Alternatively, the reverse could happen where what was supposed to be a small action kicks into a major battle with dozens of fighters jumping in on either side.
Why This Works?
I find these sharp tugs, rather than full twists, work better because not getting what you expected can be a turn off for some people. You ever see someone try a new bit of food, then make a weird face when chewing it? You ask if they like it, and they respond "not what I expected" and don't touch anymore of it. That same person, a week later, may try the same food again and find they rather like it. Why? Well, they know what to expect now.
When you're hungry and expecting chocolate you don't want to bite into sushi. Sure, both are great, but neither does anything to fill the craving for the other. However, if you want chocolate, come for chocolate, and instead of a hershy bar you get a slice of chocolate cake, most people are rather happy with the change.
So give your players cake. Let them enjoy it.
How Often To Employ
Now the trick is you can't do this all the time. It is a novelty item, a spice, a rich dessert. It's good now and then. It's good here and there. Do it all the time though, and you'll ruin the joy of it. Worse yet, you could make the players tire or even grow to hate it. Sometimes I want my bar brawl to just be a bar brawl, and not a sudden metaphor for the siege against the Dark Emperor's capital city.
Core Conflict Sticks
The other thing to remember is that you need to keep the core expectation honest. If players are expecting combat, looking for combat, they should get a fight. If they're looking for something that takes cunning to get through, they should get that. We're not changing the tools the players want to use. We're just changing how they use the tool to get the desired result. Hammers have nail claws for a reason, after all.