I'll share my stupidity. I'm pretty sure a helmet saved my life in a motorcycle wipeout 30 years ago, so I always wear a helmet when biking, except for one time - was going to walk a mile to the gym to workout, but it was about to rain, so I hopped on my mountain bike to save 5 minutes and get there ahead of the rain. Just one mile so I didn't really need the helmet. A few drops started to fall, but the road was still dry, except this was the first rain in 6 months and those few drops combined with 6 months of accumulated oil slick on the road surface. Making a right onto the main street there was a nice open gap in traffic so I sped through the corner to take advantage of it. Next thing I remember there were several people standing around me, helping me up and asking if I was ok. I must have been unconscious for several minutes. Probably 99.9% of the time you can leave the helmet behind and be fine, but it's not possible to predict which ride you really need the helmet.