J.J. Watt -- Slips On Box Jump ... Takes Out Spotter

Here's 6'5", 290 pound NFL superstar J.J. Watt trying to land a box jump during a recent training session.

The tiny man behind him was tasked with catching Watt in the event he slipped and fell during the jump.

Poor guy.

Watt tried to make the leap by jumping more than 5-feet onto the mats ... but missed -- sending himself and the spotter crashing to the ground.

Maybe Wesley Snipes was right ...