I have more times than I care to admit to. Basically operator error.The core has to be snugged down just right, which doesn’t require much torque at all. Too loose and it can fly out. Too tight and the tiny O-ring gets crushed. The core can take off like a micro rocket never to be found. So I always carry a spare core in my patch kit. Or just take the core from you spare tube and add air. No taking the wheel off and changing tubes.

Zipp makes a cute little multi spanner designed to remove cores and install valve extensions. They recommend wrapping the core and extension with plumber’s Teflon tape. If I’m at home and paying attention I do that but not out on the road. Mostly I use the pliers on my Leatherman Squirt. All this hassle my drive you to drill your rims for Schrader valves, but wait until your 50 Km out and met a stranded cyclist with a broken valve stem. Dinner and beer will be offered!