[netcdfgroup] What does nc_open returning 24 mean?

I'm having a hard time searching for this on the mailing list archive
and Google due to the unhelpfully constraining nature of a number and

I am running a program that reads in data every 'month' and it goes
through many years and then mysteriously returns non-zero after
opening the same files dozens of times.

Every time prior (again dozens) it returns zero. The file is being
opened read-only, no changes are being made to it.

Debugging info from my program:
nc_open status = 24
filename = dat/ts.nc

Nate Nutter