The Murray I recently tuned up came to me with a flat. Luckily, the owner planned ahead and bought a spare tube ahead of time, as the only Schraeder valves I have in my fleet are in the 20" diameterway. But when I opened the box, I had an "oh crap, she bought the wrong tube" moment. The valve looked like this:
As anyone who's worked in a bike shop will tell you, the quick and easy way to figure out which valve someone needs is to ask if it's metal or rubber. Metal equals Presta, rubber equals Schraeder. So when I saw threaded metal with a valve stem nut (a.k.a. "dork-nut"), I immediately thought I had a Presta valve. Checked the box, it said Schraeder. Checked again, still Schraeder. Had a momentary thought that someone at the factory put the wrong tube in the box. Scratched my cro-magnon-esque brow.
Then I looked again at that valve. Diameter's too big for Presta. And the core is down inside, not exposed. Holy schnikeys, that's a metal Schraeder valve! And now that I know they exist, I kinda want them for my 20" diameterway folding bike, since I run those tires fairly low pressure and often find myself shoving the valves into the rims while trying to get my pump head on them. It would be nice (in a "my life is too easy if these are the things I worry about" way) to be able to thread a dork-nut on there.
(The tube is from the law firm of Specialized, Specialized, and Specialized, by the way -- which makes me think that they've patented the metal Schraeder valve so they can sue the makers of Presta valves.)