Have any SHA-1 collisions happened?
In order for a collision to happen "by chance", about $2^{80}$ hashes need to have been computed. If 1000 hashes were computed every millisecond, then this would take 38,334,786,263 years. So, I doubt that this has happened by chance. 

Having said that, I would be very surprised if large government organizations do not already have SHA collisions (using better collision finding algorithms).