You never know when you're going to get an opportunity.

You never know what you're going to get as a receiver.

You never know what events are going to transpire to get you home.

Opportunity knocks only once. You never know if you'll get another opportunity.

In politics, you never know who's going to die, retire, or - in Illinois - get indicted.

I know it's so 'in' to be blase, but that's not going to get you anywhere.