I cut off an ambulance once. And yes, it had the sirens going and everything. It was leaving the hospital, and I was coming up too fast. I was afraid that if I braked, that I'd get rear-ended (in retrospect, I almost certainly would not have). So I blazed through the intersection, cut off the ambulance, and almost caused an accident.
I felt like a proper douchebag after that. I'm just lucky (and thankful), that:
1. I didn't cause an accident
2. There weren't any cops around to see that (I would have gotten a ticket for reckless driving if I was LUCKY)
3. That the ambulance was LEAVING the hospital rather than GOING to it, as forcing an ambulance to break hard with a patient inside would have been something I'm not sure I could have dealt with.
As it stands, it's still something I regret to this day. Doesn't help that I pass that hospital every day on my way to work... and I work at a DIFFERENT hospital.