A few years back I was training to ride the Seattle to Portland in the 1-day format. Had a training regimen I put together and was steadily progressing and building endurance and strength.
One day I go on a training ride on a very cold and rainy December day in Seattle. Decide to take the trail that rides through the train yard then pedal up to the docks.
As I'm riding along the trail there's this segment that crosses some train tracks at an angle. The temperature is also colder here next to the shore than by my home and the rain here is mixed with snow.
Having ridden this segment dozens if not a couple hundred times in rain or shine, I didn't think too much of it. However the added mix of snow and colder temps caused those rails to become the slickest surface known to man.
I come up to the gentle curve at speed, my front tire hits the rail of the track and my bike goes sideways while I continue moving forward. I land in puddle on my right side and then the water acts like a slip-N-slide on the blacktop and I proceed to slide along the trail at speed and slam head first, which deflected to the side, then followed up to my right shoulder, into a bollard.
I was wearing a helmet when this happened and I still laid dazed in that cold ass puddle for a good 2-3 minutes before standing up, pulling myself together, and called my wife to pick me up and take me to the ER to get the confirmation that I had a concussion and to rule out anything worse. My shoulder was also bruised and sore for a good week.
Even with the helmet, I had memory issues for 6-10 months.
If I weren't wearing a helmet I'm not sure I'd be here telling this story. If I were to have survived I would have had some serious trauma and recovery if not permanent damage.