[ozy]

It isn't: https://en.wikipedia.org/wiki/List_of_genetic_codes

On the other hand, once a certain layer of functionality exists, and starts being used to build on top of, that layer will resist change, because any variation will throw off so many higher level processes it's unlikely to be viable.

[yourapostasy]

The ultimate technical debt if there ever was one for our species. An "oopsie" at this layer would be pretty insanely difficult to architect and engineer around.

[phkahler]

Selenocysteine is a really strange one.