Luden's Echo bug!
So I'm 99% sure this is unintended by Riot, but Teemo's E doesn't proc Luden's passive. It's an on-hit DOT just like Fizz's W, which DOES proc Luden's even while inactive. Brand's passive also procs Luden's. Teemo's E deals magic damage, and takes up an entire active ability slot, so it should really proc Luden's too.