I found a couple of fun tricks you can do with this (for some definition of "fun" anyway).

Go's http.Redirect function allows non-3xx statuses, and also renders a trivial page with a status message and link:

https://301party.com/451?url=javascript:alert(%27hello%27)

Alas not infinitely recursive but enough to make your browser give up:

https://301party.com/301?url=/301?url=/301?url=/301?url=/301...

[edit to add:] https://301party.com/0 causes a panic