The dawning realisation of what a character has really been up to for so long, right under the noses of the PCs without being noticed, is one of the finest experiences in roleplaying.

For wider application, have the villain, the big bad the PCs have been chasing for eight adventure sessions, turn out to be one of the people they trusted all along. The police comissioner. The tough-on-crime district attorney. The general who ordered them on the mission in the first place.

That is, of course, if your players aren't ridiculously genre-savvy and suspect the most innocent people right from the beginning. If they do suspect kindly old Aunt Harriet of being the supervillain, you're fully within your rights as a card-carrying member of the Devious GM's Guild to pull the old switcheroo and have her be entirely innocent! That should baffle them well and good!

Transcript

Luke: Can we try another bar? I still need to get some blue milk.
Obi-Wan: The blue milk is not as important as you might have been told.
{beat}
Luke: Those people in the bar, what was their deal with moisture farming?
Obi-Wan: What do you think?
Luke: It's just farming moisture from the desert air.
Obi-Wan: Don't believe everything your "parents" told you.
Luke: But they always said...
Luke: Huh. Now I think about it, it is kinda... ridiculous.
Obi-Wan: You've taken your first step into the real world.
Luke: So... I was raised in a secret desert compound, helping to farm... what?
Obi-Wan: You're a smart kid. You can figure it out.
Luke: Whoa. Wow. Just... whoa.
Luke: So those red pills weren't vitamins either.