Remember kids... always wear a helmet. (The almighty bikeradar helmet thread)

I cannot imagine regretting wearing a helmet but I can imagine the reverse.

I can, it is when you come of your bike at some speed and slide along the road, the helmet causes your head to twist and snaps the brain stem.
Fatally.
I would regret that.

(apparently most fatalities caused by "head injury" (as described in a post mortem) are not a bash to the skull but snapping the brain stem. I was told this at an inquest involving a cyclist by the pathologist who sat at my table at lunch, most of which I left)