Valve Stem Failure

I was adding air to our bike tires the other day. Just as I had finished with the last few strokes on the hand pump I heard air escaping. I could see "Slime" oozing out around the base of the valve stem. This is the 3rd bike tire I've lost this way! Maybe it's due to the heat this past summer? The only other reason I can think of is manufacturing error but I doubt that because the tubes weren't all from the same manufacturer.

The failed valve stem.