My best WTF moments? I have one that rivals on the impossible that happened on Soulsilver.
I was going up against Red for the first time. I had Feraligatr, Flareon, Heracross, a shiny Metagross, Dragonite, and Tyranitar. In other words, I was freakin' prepared to DESTROY Red... Until I realized all his Pokemon got a level boost and new moves since GSC, which was bad since my team was in its mid to late level sixties. Add to the fact that I figured I wouldn't need any healing items, I actually thought to myself it would all be okay... Until, that is, his freakin' Pikachu OHKO'd my Feraligatr with Volt Tackle, turned around and got a critical Thunderbolt on Flareon, who had somehow missed when she used Overheat (I knew I should have used Return!), Heracross comes in, misses with Megahorn, and is OHKO'ed with a critical Volt Tackle! At this point, I am about to throw my DSi at the wall, all the while screaming "HAX!!!!!!!!!!!!!!" Knowing that wouldn't solve anything, I kept on playing. Lucky for me, I gots a hidden weapon, my Tyranitar, who knew Earthquake! :D Plus, he had a Jolly Nature, and he was ev trained in speed! To say the least... I OHKO'd Pikachu easily... Then came out Blastoise, who I decide to use Stone Edge on. Guess what happened? IT FRIGGIN' MISSED! Blastoise then used Focus Blast, and of course it hits, knocking out Tyranitar in one hit... No matter, I have two more powerhouses left, Metagross and Dragonite! So, I sent out Dragonite, who knew Thunderpunch... But alas, it turns out that Red's Blastoise outspeeds my Dragonite. Guess what he did? He used BLIZZARD (which, by the way, has 100% accuracy in hail)! ARGH! So, my shiny and glorious Metagross comes out... I know, I should have turned the game off, but I wanted to see this thing until the end. He too knows Thunderpunch, which I tap knowing I'm going to lose... Just then, his Blastoise uses Hydro Cannon. I just knew that was it, but something happened... It missed. THE FREAKIN' MOVE MISSED! At this point, I laugh at that thing's face as Thunderpunch hits it, criticals, and OHKOs that sucker. Then came Charizard, who I just knew was packing Flamethrower and was gonna destroy Metagross, but no it doesn't know Flamethrower. It does know Blast Burn though, which it uses. Guess what? IT MISSED! Plus, I had chosen Thunderpunch again with Metagross, and he OHKO'd Charizard! I am cheering my head off at this point in time, because I just thought that I might have a chance. Venusaur comes out. BOOM! Outsped and OHKO'd with Zen Headbutt. Snorlax rolls in. BOOOOOOOOOOOOOM! Outsped and OHKO'd with Hammer Arm. I get a drop in speed, but what do I care? I am on the roll of a lifetime here! Then comes in Lapras, who I scoff at and prepare to do another Hammer Arm. Lapras moves first... And uses Body Slam. Evidently, it was not my day to defeat Red, as the move paralyzed Metagross, who couldn't use Hammer Arm thanks to Lapras. Lapras then uses Blizzard, which made Metagross go into the yellow, a good bit below half. Metagross finally hits the Hammer Arm though and just as when I thought I was going to win, Lapras' health bar stops at...one. One. Health. Point. Next move, I figure he's gonna heal, so I decide to use Hammer Arm again. Meteor Mash was too risky. Guess what? HE DOESN'T HEAL! Instead, Lapras uses Brine, effectively knocking out my last Pokemon and destroying my chances of the greatest comeback EVER!
So yeah. That's my greatest WTF moment from the 4th Generation. Say what you will, but I am a firm believer in that if the game decides you need a good ol' fashion butt-kicking, thou shall receive said butt-kicking.