Bike Helmets.

Do bike helmets make you safer? Not according to the British Medical Journal.

[I]n 1988 the largest survey of cycling casualties ever undertaken concluded that helmets did not prevent injury; indeed, increased use correlated with increased risk of death."

Why? Possibly because:

The likelihood is that when wearing a helmet cyclists feel less vulnerable and therefore ride less cautiously. As a result, they are more likely to have an accident. Consequently, the benefits attributed to helmets by the studies are at best highly exaggerated. At worst, wearing a helmet may expose cyclists to greater danger.