I saw an anime on Adult Swim, probably between 2002-2005. It might have been the first episode. But the main character has amnesia or had his memory wiped. He was carrying an acoustic guitar and someone on the street says "Do you play?" and he says no and then plays a really intricate riff and the guy replies "Lying is bad karma, man" and it's obvious the main character doesn't know how or why he can play guitar.
Please know that my quotes are probably paraphrasing the real dialogue.
The anime was in Japanese style but the audio was in English. Also, it was not Phineas and Ferb, I'm familiar with that show.