This just happened. My 4yo son wanted me to carry him to bed. I obliged as I often do. Also often I tell him he's getting so big that some day I won't be able to carry him. But today was different.
Inspired by one of the most memorable posts I've read here, as I laid him down I said "one day, unknown to either you or me I will put you down never to pick you up again."
I thought it was going to go right past him like when I say he's getting too big for me to carry.
Instead, he looked at me from his bed, tears welling up in his eyes, and said in a quiet little voice, "Daddy, that's so sad."
I lost it.
We both wept for about an hour until I held his little hand as he fell asleep.