The Castelia Sewers seem to have been a lucky place for me in Black 2. One time I was training my Pokemon there and encountered a shiny Rattata. Another time, I was training there and went back to the Pokemon Centre, and noticed the medal guy was there. I was thinking, "What did I get a medal for?" And I literally jumped in my seat when it turned out to be the Pokerus medal. I spoke to the nurse and sure enough, my team had caught Pokerus training in the sewers. I totally didn't expect both of these things to happen so early in the game (I only had two badges at the time).
Speaking further of Castelia City, I remember an unexpected moment way back when I battled Burgh in White. I was up against his Leavanny (also his last Pokemon), and sent out my Blitzle for the purposes of paralyzing it. So Blitzle used Thunder Wave, and barely survived Leavanny's attack with 1HP... I decided why the hell not, I'll use Flame Charge on it. Did that, and Leavanny was fully paralyzed. Next turn, Burgh used a Super Potion (or some other healing item) on Leavanny. Blitzle took out ~1/3 of Leavanny's HP with Flame Charge again. Turn after that, another Flame Charge, Leavanny fully paralyzed. One more Flame Charge, and Leavanny fainted! I had brought along a few Fire/Flying/Rock-type Pokemon for taking care of Leavanny, but I never expected my Blitzle to win that battle!
In the same White game, you won't believe how loudly I cried "OMG!" when I caught Terrakion in the first Dusk Ball I threw at it. Granted, it was already at 1HP and asleep, but still...
I also get a little annoyed when a trainer's Pokemon uses an unexpected TM move. My biggest example of this was when I was progressing through Victory Road in White, and decided to send out my Jellicent to battle a trainer's Beheeyem. First turn, Beheeyem used Thunderbolt. Not only did I not expect that, I literally thought the trainer had specifically designed that Beheeyem to be good at dealing with Jellicents.
And finally, back to the Castelia Sewers in Black 2, I still remember how shocked I was when I entered there early this month and saw that all the water was gone.