Many posts here are soliciting information, recommendations, etc. This one is more a statement of one experience from last week.
Riding my normal daily route two of us had stopped at a stop sign, waited for traffic, then departed onto a busy street. I pedaled two or three strokes then stood up in a tall gear and hammered to accelerate to around 20 mph. Nothing unusual at all. A clear, beautiful morning the same routine that I have had 3-4 days each week for 10 years. All I remember is thinking, " this is going to hurt," as I slammed my head into the pavement. The impact knocked me out for awhile and my riding partner who heard the impact came back and got me and my bike up out of the traffic. After several days I have pieced together what I think happened. My right shoe came unclipped on the downstroke and the shoe hit the pavement with the toe bent underneath, the chainring impaled itself in the back of my leg. I am not sure of the physics of the rest, only that my helmet (the styrofoam) broke through at impact, and I left a pound or more of flesh at the scene.

I am not trying to sell helmets, nor do I care if you wear a helment. If I had not had a helmet on, my cranium would have fractured from this impact. The helmet left a large bruise on my forhead as it was. Two days later I went to play tennis and discovered that my right arm was not functioning properly. The impact had put a lot of pressure on my neck and spinal cord which caused swelling etc. even though it wasn't really painful. My arm has come right by now after several days on the tennis court. I have not been on a bicycle for a week.
Just saying, be careful out there.

I hate that "this is going to hurt" thought, as things are going south.
I'm glad you survived, hope you heal well and soon.
Blessings

Ouch! Hope you mend quickly.

+1 Heal up and get back out there.

Heal quickly!
What clipless setup failed (brand name, model)?

I know a local rider who lost a pedal on an out of the saddle sprint just like you did. He was injured even worse. He spent some time in critical care in a level 1 trauma center.

It is easy to have happen. I hope you manage to heal up and put it behind you.

Never hammer. Always go slow. Enjoy being crushed. Flow with the Tao. Eat your broccoli.

Seriously, glad you could walk away from it. The time one has between "Gosh, I'm flying in the air and if I recall the laws of physics, I'm going to have to impact upon the pavement sooner and later" is almost as unpleasant as the actual impact.

And use sunscreen. Don't forget the sunscreen.

Sort of know the feeling. Today I am cruising along and some teens is a little black car threw a plastic container of Armor All wipes at me and my luck with these incidents ran out. The thing made a direct score in the small of my back and I was headed for the gutter. Managed a safe landing and no damage to either me or the bike. Called police but that was just a waste of time.

So even though the thing only weighed about a pound it was traveling at 50+ mph when it struck and really hurt. You gotta wonder, these morons are most likely old enough to vote.

I hope you at least kept those wipes. Ya gotta get something outa the deal.

Nope, the cops took the container as "evidence". I have full faith that it has been sent to the CSI lab and an arrest is due shortly!

Back in my youth I was riding along on a highway and suddenly a car passes and WHAM! I'm knocked off and into the ditch on the side of the road. Fun kind of guy threw a pint of milk and scored a direct hit on the back of my head. If it wasn't for the milk every where I would have not known what hit me! Hurt!

I started wearing a helmet in 1990. Broke two badly and one ruined by a slide. I'm a believer.

Thanks everyone for the good wishes. For Sojourn: I have been thinking a lot about the "coming unclipped" process and how it happened. It may well be that I never completely clipped that shoe into the pedal after I started. Those pedals have 10K+miles and the shoes/cleat even more. I have the adjustment set to easily unclip. The pedals are SPD/Shimano and I do not believe that they were the cause of the unclipping. Too many other factors involved, including my complete disregard for something as simple as getting a solid clip-in before standing up. Cheers to all.