I built my house in 1999 and I've already had at least 3 of the roof vents on my house start leaking. This is because the rubber seal used on the flashing dries out in the hot sun and cracks allowing water to penetrate the opening left where the seal once was. To fix this in the past I've had to hire a roofer to come out and replace the flashing because it involved getting the flashing out then back under the shingles. The one time I tried to do it myself I just messed the shingles up. Today while looking around Active Rain a little bit I came across an article about a simple yet effective fix if this happens to you. Check out the website http://creativecomposite.com to see what they have to offer. I haven't used the product yet myself but you can bet I'll be trying it the next time I need to repair a leaky roof vent.
Check it out for yourself.