“Never interrupt someone doing what you said couldn't be done.”
“Never interrupt someone doing something you said couldn't be done.”
“Never interrupt someone doing what you said could not be done.”
“Don't criticize someone doing something you said couldn't be done”
“Nothing is so embarrassing as watching someone do something that you said couldn't be done.”
“The world is moving so fast these days that the man who says it can't be done is generally interrupted by someone doing it.”