Catch somebody on the hop
Definition 1 of 1
Pro Tip 1/3Readiness
The phrase emphasizes someone's lack of preparation for a situation or event. 
When the pop quiz was given, it really caught me on the hop.
Pro Tip 2/3Surprise Element
Implies an element of surprise that can cause confusion or disadvantage. 
The sudden inspection caught the staff on the hop.
Pro Tip 3/3British Origin
This phrase is primarily used in the UK, so knowing your audience helps avoid miscommunication. 
As an American, I was caught on the hop by their British slang.