As all fans of How I Met Your Mother know, a slap bet is a bet where the winner gets to slap the loser as hard as they can at any given time in the future.
Zane had a credit of one slap on his buddy Steven, and decided to cash it in at the least expected time. At his own wedding, during the ceremonies.
While giving all of his groomsmen a hug, he finished his hug with Steven with a nice, loud slap!
Robert Montgomery caught the whole scene on camera, and posted it online way back in 2012. The video has only exploded online now, even appearing on Gawker, 22Words, and HuffingtonPost.