catch someone off guard

(verb) to surprise someone in a good or bad way

Example Sentences:

My boyfriend caught me off guard when he pulled out a ring and asked me to marry him. I was so surprised!

Our boss is always catching people off guard and asking them to work on weekends.

A: Hi!
B: Oh my god!
A: Sorry, I didn’t mean to catch you off guard.
B: Oh, that’s okay.

From the Blogs:

NYPD caught off guard as leader of new patrol group has shady history

Media:

Similar idioms:

1. shady
2. catch someone red-handed
3. catch on
4. catch you later