How to fix error `cached package a-x.y.z: size mismatch, cannot continue` during `pkg upgrade`?

[Coert]

On Wednesday 25 February 2015 05:06:32 Karl-Philipp Richter wrote:
> Hi,
> According to `man pkg` `pkg clean` should fix the issue, but doesn't. Is
> that a bug? Experienced with `pkg` 1.4.12 on FreeBSD 10.1.
> 
> -Kalle

Try deleting either everything or just the offending package from 
/var/cache/pkg/

-Coert