I went through this phase where I endoed on the first ride on a new mountain bike. This happened on my Gary Fisher and I still don't know what caused it. The landing was soft and we got a nice laugh about it. However, when I got back up and started rolling again, about 5 seconds after getting rolling I can over a stick. It broke under the weight of the front tire, the right side of the stick shot up just as my right leg was on the down stroke and the end went into my shin. Thankfully the bone stopped it from going in very far. About an hour later I was able to pull out a chunk of wood about a half inch in length and width from the middle of my shin. It left a nice scare that always gets a grimace from people that hear the story.