This is pretty horrible to watch, so
don't watch it if you're sensitive to crash videos.
The rider reports only suffering a double-fractured ankle from this, but it's pretty scary to see the crash and hear him in pain after.
Link to original page on YouTube.
I personally think it's good for riders (myself included) to see videos like this.
This is a good reminder why we wear gear. This guy seemed to be wearing a helmet and gloves but no other proper protection.
This is a reminder to practice emergency stops. This guy lost control in a panic.
Generally it's a good idea to ride offset from the car/bike in front so you have the option of making a slight steering adjustment and overshooting.
Also this is a reminder to pay attention at all times. He didn't hear the fire truck but he should have been scanning the roundabout for dangers and visually noticed it. The green light doesn't mean it's safe to go.