Forgot my helmet once riding to volunteer at the bike co-op (less than 3 miles from home). It only happened because I took the Trek that day so I could do some work on it during adult shop time, that bike is kept in the basement, and the helmet was with the Surly which is in the kitchen and is the bike I usually take. So, I brought the bike up and forgot to go back upstairs to get my helmet (oops). Realized it when I was more than halfway to the shop so kept going, but was embarrassed to be setting a bad example by arriving there without my helmet!