Catch Someone On the Hop

Meaning:
It means to catch someone when they are not expecting it.

Example:
Tim thought he could skip school by feigning illness, but he was caught on the hop by the principal who spotted him at the arcade.

Did you know?
The hop is a flower used to brew beer. Back in the olden days, hop-picking was an activity that men often had to find an excuse to do. Occasionally, their employers would discover their lies and catch them picking hops, literally.

Reference: http://www.englishdaily626.com/idioms.php?012