This answer mentions that the C. elegans hermaphrodite has exactly 302 distinct neurons. This has made it a very effective model for a variety of types of biological research, including neurology and cell differentiation. It is also currently the only organism with a completely mapped connectome.

But the word "always" made me wonder - has a viable specimen ever been verified to naturally have a number of neurons other than 302? Not as a result of an experiment, just naturally?