I almost ruined a really nice old Yashinon 50mm lens today by taking it out on a walk in the rain.  I knew it was a bad idea but I'd just got an adapter to fit it to my camera and I was in the mood for some manual focus action.  It was pouring when I left the house but I thought I'd keep the camera in the bag if the rain continued - which it did, until long after I'd got back.  Of course I kept seeing things I thought would be worth capturing so it kept coming out of the bag.  A large splash of gritty rain water at some point landed on the lens and filled the focus mechanism with sandy deposits, which then felt like a pepper grinder.  I had a waterproof jacket on but my jeans (not a good idea) got so saturated that the water traveled up them by capillary action, under the jacket, into my T-shirt and eventually got to the inside lining of the waterproof jacket and the pockets filled with water from behind. 

Back home, lots of swearing, wiping, tapping and rotating of the focus ring, followed by just leaving-it-the-hell-alone for a bit seems to have cured the lens of lasting damage.  The general advice for outdoor use is that unless your camera (and lens) is of the expensive weather sealed variety you shouldn't use it in anything more than light fog.  I subscribe to the Angry Photographer's advice (look him up on Youtube) that there's no such thing as a water proof camera.  If you regularly expose your camera and lenses to rainy weather eventually insidious water and dampness will make its way in and start to cause corrosion to internal elements.  I'll be more careful in future.