Edit: Since wikipedia doesn't have an entry for "shaliach mitvah" I feel like I should define it.
There's an idea that you won't die while in the process of carrying out a Mitzvah. To that end, people who are traveling are often given a dollar for charity. The idea is the the recipient, now being the process of giving charity, is protected by the dollar.
In DC this weekend, my homegirl Nechama gave me such a dollar to bring back to Boston with me. Little did she know I was already protected by not having paid off my blogathon debt. Anyway, I added the mitzvah dollar to my blogathon payoff and now I'm entirely without mitzvah to protect me. So now is not the time for me to cross the street without looking both ways.