catch someone on the wrong foot—surprise a person when one is not ready or expecting smth.:
- I’m afraid you rather caught me on the wrong foot, asking for it at such short notice.
put a foot wrong—(usually negat.) make the smallest mistake; make a blunder:
- He complained that if he put a foot wrong, the manager was down on him at once.
See also: get off on the wrong foot / get out of bed with the wrong foot foremost.