flat-footed, to be caught/catch

To be startled or be caught unawares and/or while unprepared. The thieves were caught flat-footed by the security guard.

If someone is caught flat-footed ,they are put at a disadvantage when something happens which they do not expect. `The people around were caught flat-footed,' said Mr. Enko. `Nobody expected floods of such magnitude.' Note: You can also say that an event or action leaves someone flat-footed. Pentland had agreed to buy Adidas but pulled out of the deal suddenly, leaving the French millionaire flat-footed.

To surprise/be surprised; to be caught unprepared. This antonym to being on one’s toes is believed by some to come from baseball terminology (it was so defined in the linguistics journal American Speech in 1912). However, other authorities believe it comes from horse-racing, where it is said of a horse whose jockey is unprepared to start a race.