When some errant drops of windshield washer fluid from the car in front of you land on your own windshield, thus allowing an ineffective yet "free" cleaning.
Jim: "I really need to refill my windshield cleaner. I have bird shit caked on here from last week."
Mike: "Just keep tailing that SUV and you might stumble into some serendripity."
Jim: "Seriously, Mike?"