New warning, what does it mean?

I'm getting the following from my main disk on boot now:

GEOM: ad0s2: geometry does not match label (255h,63s != 16h,63s).

What does it mean?  This system is a dual-boot FreeBSD/Windows
laptop.  In the past I've seen sysinstall barf on the former geometry,
and have had problems installing dual-boot core4/FreeBSD systems
because of it.

In addition, the above error message is useless.  Who is complaining?
Why are they complaining?  What is the corrective action to be taken
here?  Why does it matter with modern disks and BIOSes anyway?

Warner