Happened in episode 244.

The group's doing a meditation class where they try to "think of nothing", but comically fail. Ash thinks of becoming champion, Misty thinks of losing her "legendary water pokémon" (really, that's what she identifies it as), and Brock thinks of something… a little naughty. It was cut from the English version, but he starts fantasizing about chasing a woman. But not just any woman. A… particular kind of woman.