Fighting has always been part of the game of hockey and it always should be. Fighting allows players to settle grudges, control the game and police one another. However, anytime you drop the gloves there is an inherent risk that players accept.
When two players decide that they need to settle their differences, it's up to the linesmen to ensure the safety of both players. Unfortunately in the following clip a linesman makes a rookie mistake that ends up ending with a player being knocked out.
As these two players square off and prepare to drop the gloves, one of the linesman makes a very dangerous decision. Instead of circling the two participants the linesman cuts directly in front of the two. This choice leaves one player's vision impaired and leads to the impending right hand and as he steps forward he eats a shot directly to the button. The left hand sends him directly to the ice.