I recently purchased 50mm tubular wheels and had to put valve extenders on my tubes. I ended up getting the kind where you don't remove the original valve core, and they have an extension for the "valve button" that surrounds the original one.
In tuesday's race, I ended up with a slow flat. I ended up having to retire, and started to replace the tire yesterday. As it turns out, the leak was caused by my unscrewing the "valve button" extension too far, which then proceeded to unscrew the actual valve core. I didn't screw it back in far enough and voila, a slow leak.
Kinda sucks. I'm putting on new valve extenders where you move the actual core.