catch with pants down

catch (one) with one's pants down: Fig. to discover someone in the act of doing something that is normally private or hidden. (Literal uses are possible.)

• Some council members were using tax money as their own. But the press caught them with their pants down and now the district attorney will press charges.

caught with your pants down: in a situation that embarrasses you.

• My job was to get as much information as possible so that my boss would not be caught with her pants down.
  
  Usage notes: often said about situations you are not prepared for

... http://idioms.thefreedictionary.com/pants