When, somehow, a little bit of shit ends up on the underside of the tip of a penis.
When a man sitting on a toilet after takin a crap wipes his ass from back to front. And without realizing it brushes his soiled tissue under the tip of his penis. As his partner later on unzips and strips him, he or she, while looking at his unit straight in the eye, notices a 5 oclock shadow.
When you purposely walk in on your brother jerking off while your jerking off.
"I had that Paris Hilton video going in my room while I was cranking one out, when my older brother bill-dog walked in on me while he was beating off and laughing like a maniac! Bill-dog hit me with the worst 5 o'clock shadow! I was so embarrassed!"