Have you purchased a race bib from someone else? Have you sold a race bib? Have you run bandit in an organized race...
I've sold one bib. It was for the Carlsbad Marathon. A turn of events two months prior to the race left my training in ruins so I sold (it was actually transferred to him) my full bib and re-registered for the Carlsbad HALF Marathon event. I lost a little money on the deal, of course, but the half was definitely better for me at that time.
I purchased a bib last month for the Rock-n-Roll San Diego Half Marathon. A group of women in San Francisco had planned to run together, but their training was lacking so they sold their bibs. It was actually for the full marathon, but RnR events have a great option of allowing racers to change to the half marathon event race morning (you simply run the half event instead and your time is accurately recorded -- does not work to run the full with a half bib though). I purchased her bib for almost 1/3 what she paid. Transfers are not allowed so I ran the event as her.
Last night I received confirmation that my name has been registered for another half marathon event next week. Again, I purchased a bib from someone who is unable to run and this event allows transfers so I will be running as me. And I can't wait! I saved $25 off the registration price and will get a shirt.
As the prices of these events increase, I'm really liking the ability to purchase bibs from others -- whether or not they are transferred to my name. Really, the only race event I care about having my name attached to is the marathon when I finally get a BQ. Until then, I'll save a few bucks and have fun running some races!
I may try to get a bib for another half marathon in August, too. :)