My black lab puppy has taken someone off a bike, no collision though, the guy was startled by him running out of a bush and lost control. Luckily he didn't have a five grand bike and was unhurt.
Trying to get him used to bikes now by taking him out with me cycling, he doesn't seem to understand that in front of wheels=ouch.
This is a similar reason to why I've recently ordered a helmet, commuting most of the way through rural Hertfordshire, I'm constantly on the lookout for rabbits and badgers flying into my path.