TYPO3 Core

The site module states:

Make sure to have at least 0 (not defined otherwise) configured in order to serve helpful error messages to your visitors.

however there is a strict comparison with the configured error codes which means that if e.g error 500 and 0 are configured but a 404 occurs, it is not handled.

checking 0 makes it easier for simple site setups having just 1 error page because only one configuration is needed.