Simple rookie mistake, kind of like riding to the store without your helmet. You don't think anything bad will happen but you forget that you can't determine your future.
For added info, this guy should not have used WD40 on an o-ring chain in the first place. WD40 will ruin the o-rings between the chain and cause them you split apart which you will not notice until your chain becomes rediculously hot and expands the links until it breaks apart and whips you in the leg.
Best thing to do is take it for a 5 minute test ride and slow speed to warm the chain up and then spray the chain down with a chain wax and wipe off the excess WHILE THE TIRE IS NOT SPINNING.