I wanted to find a Lapras under the Village Bridge in White 2. I read online that it was a 5% chance of finding it in rippling water. So, I bought 78 Max Repels and started. I kept hoping that I would find one, however, I couldn't. Once I said, "Dang, I'll never find it," next thing you know, BOB'S YOUR UNCLE THERE IT IS. I mighty beauty, if I do say so myself. However, guess what it used?
You guessed it. Perish Song. ;(
But I eventually caught one after the next six times I found them.
Also, when I found a shiny Bellsprout in my Pearl version. I almost died when I found it. The problem was, though, that the Bellsprout was a level 20. All my Pokemon were level 60+.
I eventually caught it, though. On my last pokeball. THAT was scary.