I had a different presta issue the other day; I had been dealing w/ a very slow leak on my front tire (very slow, just pump it up every week or so), and then Thursday, after I pumped it up that morning, it was flat by lunchtime. I had a crappy floorpump at the office that required a presta-schrader adapter (that's not the crappy part of that pump), so I put it on, pumped it up, and then, when I was taking it off, the core of the presta valve, w/ the adapter attached, shot across the room like an airgun pellet.

Apparently the threads holding the core into the valve had been loose, and I was maybe loosening it more as I was pumping it up, and then I loosened it a lot while removing that adapter. Anyway, I was glad I could thread the core back in, pump it up again, and ride home (and it's still holding pressure now).