Take Marc Staal of the New York Rangers, he encounters Vancouver's Alexandre Burrows with some pushing and shoving in front of the net. Both parties are playing for space, but Burrows takes it up a notch when he slams his stick into Staal's junk.
Not happy with that, Staal retaliates by tripping Burrows and gets whistled for the penalty. He then was forced, sore nuts and all, to the penalty box.
I love the announcer's final comment in the clip, "[The ref] might have missed the stick to the nether regions."