Files disappearing from /bin

[Robert Perlberg]

I'm having a weird problem with Cygwin.  I've been running the same version of Cygwin for 9 years without updating the package or the operating system (Windows 8).  Everything has been working great for 9 years, but now whenever I use the package, files disappear from the /bin directory.  Specifically, whatever executables are running at the time will disappear.  Does anyone have any idea what might be causing this?