What word does your LO totally butcher that makes you laugh every time you hear it?
My DD just learned "elephant," but it comes out elalalalala. My DH and I think it's the single funniest thing we've ever heard. She has a stuffed elephant at home and a few of her books have elephants in them, so she consistently uses the same "word" for elephant, but it's so far off that it's just too funny!!! And I absolutely LOVE that elalalalala is what she hears when we say "elephant."
When my DS learned to say "help," it came out "em," so it took us a while to figure that out, but eventually we figured it out and realized he was using "em," consistently to ask for help.