I installed some new tires on some new wheels for someone recently, and he told me the next day one of them was flat. He dropped it off, and I sprayed the whole tire, and wheel with soapy water. Looked everywhere and no sign of anything leaking. So I focused on the rubber TPMS valve stem, and saw one small bubble on the side of the stem. Normally I take the valve cap off too, and check to make sure the core isn't leaking, but this time I had left it on, since I new it was torqued correctly. Sprayed more soap on the stem, and I couldn't see any more bubbles. Figured I would just try a new stem, and when I started to remove the valve cap I saw lots of bubbles all of the sudden. There are countless styles of plastic valve caps and most of them have a dimple molded on the inside of the cap, but the dimple on the cap that was on this the dimple was big enough, and shaped to actually depress the valve core when you snugged it up. Never looked for that as being an issue ever, but now I will look at the inside of every valve cap I put on.