stumble across something
stumble on something
stumble upon something

He must have stumbled across us in the past two hours.

She stumbled across the floor to the bed and stood looking down at him.

He put down the phone and stumbled across the room.

I don't think they've ever stumbled across real life together.

It looked like he'd stumbled across some sort of army.

He rose from the bed and stumbled across the room.

Did you really think you'd stumble across her kids here?

And in the process we stumbled across a great idea, an entirely new security.

On the way he stumbled across the body of his own victim.

Looking at those letters made her feel like she'd stumbled across somebody's private place.