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# Unknown command

The Unknown Command error (421, ERR_UNKNOWNCOMMAND in [RFC 1459](https://datatracker.ietf.org/doc/html/rfc1459#section-6.1)) is when the server doesn't recognise a command a client sends.

```
↗️ UNKNOWNCMD :something
↙️ :server 421 nick UNKNOWNCMD :Unknown command
```