catch smb. napping

(off balance, off guard, on the wrong foot или unawares)

   зacтaть кoгo-л. вpacплox

    Where so much was at stake he did not propose to be caught napping (Th. Dreiser). It was Mrs. Skeffington. As she opened the door, her face was reddened, her manner flustered. She said: 'Oh, it's you two, is it? I'm afraid you've caught me on the wrong foot' (C. P. Snow)