For me it's also the small pair that makes a set. When you see three straight cards like 6-7-8 on the board, it's not too hard to imagine someone holding 9-10. After all, it is not an unreasonable starting hand. You can make a bet to see if your opponent continues. But you can also proceed with caution, practice pot control and not get carried away. The same thing goes for seeing three suited cards on the board. You know that some players are fond of playing any two suited cards and can see the possibility of your opponent having a flush. Again, you can proceed with caution and not get carried away. But when you're holding top two pair on a disconnected board where you see draws and someone is betting in to you, all you think of is all the times that some fool fired his entire stack at you with top pair and a weak kicker or bluffed off all of their chips with nothing but air. Then they turn over that small pair and you see that they flopped a set.
When people used to remark on the fact that I was constantly making deep runs in tourneys, I attributed to the fact that it's hard to win big pots against me. Part of it is not getting carried away every time I think I'm ahead in a hand. The other part of it is being able to see certain hands coming, being to put someone on straight or flush draw, and to being able to get away from my hand when I see the card that completes that draw land. It's not that hard for me to lay down Q-Q when I see an Ace hit the board on the turn, knowing how many players out there will refuse to lay down any hand with an Ace in it. But it's hard to see a set coming, especially when you see so many maniacs constantly making ridiculous plays. I would say the only way to win a really big pot against me was to surprise me by turning over a hand that I just couldn't see coming. And if you play it right, it's hard to see a set coming.