I actually didn't know there were more error codes beyond error code 429

There's "431 Request Header Fields Too Large" which you will see occasionally. But after that 451 is the only other 400-level error code above 429. It was chosen as a reference to the book Fahrenheit 451.

451 is kind of a novelty code, its meaning being related to Bradbury's "Fahrenheit 451" SciFi novel.

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