ERROR: plpgsql: cache lookup from pg_proc failed -- Why?

ERROR:  plpgsql: cache lookup from pg_proc failed

I am getting this error whenever I try to use a a plpgsql function that I
have created.  It does not matter how simple the function is, I can't get
anything to work.  I can't seem to find any answers in the documentation or
on the web.  I know plpgsql handling is installed by default, and I
certainly didn't disable it.  A quick once over of some of the relevant
code didn't provide any obvious solutions.  Can anyone provide an answer?

Thanks!
Ben