Developer: "We don't need to write documentation. Users should obviously know what to give as input. "

Users:

@davidbisset
Not sure of the attribution and I paraphrase.
"When it comes to designing something that is foolproof, your average designer underestimates the ingenuity of the average fool"

@raymierussell @davidbisset

Slight variation:

Every time you try to idiot-proof something, they invent a better idiot.