As you might have guessed from my vampire post above, I enjoy scaring my metagame-thinking players from time to time, by making them think an encounter is gonna be a whole lot harder than it actually is. Can't do it too often, or the trick will loose it's effectiveness and encounters shouldn't be an anti-climax
every time,

but it's fun to teach them the errors of their meta-gaming ways once in a while!
Twice I've scared them with dragons that they knew were too big for them to take on. Once the "dragon" turned out to be wyvern, the other one was a chimera. I figur that to the average commoner anything big, scaly and flying
has to be a dragon, right?
Assuming the PCs are good-guys, let them meet up with a starving, desperate family in the wilderness; with lots of kids and preferably carrying at least one crying baby. They've fled their small, isolated farm because of this great, terrible dragon that keeps eating all their sheep/cows/goats/whatever, and they fear one of the children might get eaten one day.
Make sure to portray the family's situation as as hopeless as can be. The dragon ate their only horse and they've only got what little they can carry with them (including one baby!). Their clothes are dirty and ragged and all the children look really skinny.
Of course, they'll beg and plead the PCs to go kill the dragon, offering them everything they have as a reward. "Everything they have" being a handful of coppers, grandma's wedding ring and their oldest daughter's hand in marriage. If you're feeling really nasty (and your players are mature enough!), make sure to describe the girl as worty, crosseyed, buck toothed and twelve years old.
They'd have to have hearts of stone to refuse these people their help. And hearts of ice to accept the "reward" afterwards, naturally...

If they suggest escorting the farmers to the nearest town to get some more able help, let the farmers tell them there isn't enough time for that. "If you hurry over there and kill that nasty dragon right now, there might still be enough left of the place for us to start over. If you don't, we're heading for a carreer as beggars..." Then have the oldest girl say "Mom say I'm gonna have to be a prostitute, but I'd much rather marry one of you handsome men!"
If they don't go dragon-hunting by then, find some new players!

Of course, the "dragon" won't be as tough as your PCs will think. Use either a wyvern or a chimera (or maybe even a half-dragon kobold!).
If they wanna marry the girl or not, is of course entirely up to them!
