Happened to me with Melee. I was the best player in the school, something I had proven time and again outside the tournament. My best character was Pikachu.
Then the tournament rolled around, I was cocky. What happened? It was a 4-way free-for-all. I was Pikachu. There were two Marths and a Fox, which made Pikachu one of the worst possible choices to fight with. What's worse, right behind me some dickheads kept telling me to use Thunder - if you've ever played a Smash Bros. game in a semi-public area, one thing you'll notice is that everybody seems to think Thunder is a spammable move and a good Pikachu player needs nothing else. So I didn't use Thunder, partly to spite them and partly because Thunder was a stupid move 90% of the time. Then came a bit when Thunder would've won me the match. I didn't use Thunder. I died.
Also happened in a Brawl tournament at this anime convention. Now I don't actually have a Wii yet, so I'm not fantastic at Brawl and I certainly didn't have a chance of winning the tournament. I fought nevertheless for giggles and shits. I managed to make it to the second round, which pleased me muchly. Perfect execution of ROB's upsmash. Second round came along, I lost, but not before the other two players (again, it was a 4-way free-for-all) had been beaten. So, I came second in the second round, without actually having had the opportunity to play Brawl that much (I'd played it a few times). I was pretty pleased with myself.