I just removed a tube that only leaked at >80 psi. It would leak down to 80, then stop. It was a small tear by the valve stem that seemed to be held together by the pressure at <80, but allowed air through when the pressure was higher.

It's from the threaded Presta tube holding on too tightly to the pump head. Smooth valves are much nicer in this regard, and I used to have some, but I don't know where I got them, dabgummit.