I have been using Continental tubes mainly because I can get them with 36mm Presta stems. But they have replaceable valve cores which I cannot understand there being any advantage.
Today I wanted to see how well a presta to schrader adapter worked. When I unscrewed it out came the value core. That was the second time a valve core came out. The first time it happened to me was when I took off a valve cap and the core came out as well.
So I'm now thinking maybe a bit of loctite red would eliminate future mishaps for me.