I've used countif hundreds of times before, but now it has stopped working:

=COUNTIF(L11:O11,"c11") produces a 0 - apparently doesn't recognize cell
C11 - note that it remains lowercase in the countif statement rather than
automatically changing to uppercase (the correct calculated answer should be
2)

No cell reference between quotes is accepted.

substituting a real number for c11 functions properly.

I emailed this to a friend and it didn't work on his Excel either, proving
that it's not an application malfunction.