More then a few posts in this thread speak of small malfunctions in flashlight operations. My suggestion for any of these, are to first take the torch apart enough, to get to all the contact points and clean them with rubbing alcohol. lube the O-rings and put it back together.

One of my Ld20s had some weird thing happen after an hour of operation, cleaned the contacts and its worked for quite some time now (~10hrs) with not a problem. I've even dropped it from about 5 feet onto concrete, just a nick but works as it should. I've also opened a few brew bottles in a pinch, well worth the paint on the tail cap.