I lost it at "tiny Typhoid Mary" 
Ds2 (3) loves to crawl into my lap to cough. Loud phlegmy coughs, an inch from my face, then look at me, all proud. I tell him, "Hand! Cover your mouth!" and he covers his mouth.. with my hand.
He once came in from the other room, wiped his snotty nose on my trouser leg then wandered back off to play. Love him to bits though.
Ds1 (5) "muuuuuu, mum, mum, muuuuuuuuuuuuum, er... I forgot." x 1000.
"Muuuuuuuuuuuuum, mum, mum, mum, muuuum, can I have a snack, I promise I'll never ever nag you again!" I give him a snack, he finishes, immediately nags for another. (to be fair, I would do the same!)
Also, if I go for a wee or god forbid a poo all I can hear is heavy breathing outside the door and tiny fingers scrabbling at the lock. "What are you doing? Are you having a wee? Are you finished? Have you got a penis? Why not? How do you wee? Are you finished? Mum? I need a poo! Are you doing a poo? Have you got lost? Mum? Muuuuuuuuuuuuuuum!"
Oh, and both children, and dh. "Where's the pen?" Me - "look for yourself" "I can't seeeee it!" Me - "look, it's right there, by your foot." "what foot" "Just look by your foot!" "I can't see it!" "IT"S RIGHT THERE!!" "Where!" etc etc.
Now I just tell them a brontosaurus ate it, tough luck.