ok this myth is an idea that came to me after mowing the grass one day.(did not acualy happen to me)
Your mowing your grass with a normal lawn mower, and you have come to the curb by the road, your lawn mower has a gas leak and as you are mowing the lawn your blade from the mower hits the edge of the curb, but some of the gas has leaked out onto the curb, the blade generates a spark and your lawn mower catches fire! You catch on fire aswell and burn to death.

if you have THAT much of a gas leak and dont bother to correct the situation,then you deserve to burn to death.

I could see it more as a spill or something when the mower is shut down, and then a blade hits a rock or something when it's started up, generating the spark.
Unlikely, but interesting to see tested.

What vlad_tepes said!

I'd figure that fuel leaking onto HOT engine parts could ignite.

There have actually been lawnmower recalls in the past involving fuel leaks.

http://www.cpsc.gov/cpscpub/pr.../prhtml08/08077.html

Gasoline leaking onto a muffler can ignite, I've had it happen to Me while filling the tank on a hot mower. However, if the mower is leaking enough gas to get a sufficiant fuel air mixture underneath a RUNNING mower, the operator deserves whatever happens as a result of it. Thats a lot of leaking gas to not notice or simply ignore.