Want to see a NBA head coach freak out? Don't watch a Boston Celtics game.

That's usually the case, as Brad Stevens is about as level-headed as they come. But the normally mild-mannered Celtics coach surprisingly broke character in the first half of Tuesday's game against the Milwaukee Bucks.

After Isaiah Thomas' failure to pick up Khris Middleton led to the Bucks sharpshooter knocking down a 3-pointer, Stevens called a timeout and ripped into Thomas as he approached Boston's bench.

You know you've done something wrong when one of the calmest coaches in the NBA is ripping into you.

Fortunately for Thomas, though, it appears Stevens' moment of fury was brief.

Just don't let it happen again, Isaiah.

Thumbnail photo via Brian Spurlock/USA TODAY Sports Images