Strange 403 Forbidden reply with a certain phrase in quotes - others get it too

[usonian]

10. There is protection against hack attacks

and some characters are apparently forbidden, as is some HTML.

As an outside example, Mister Null has real problems with computers, as it's a reserved word.

https://www.wired.com/2015/11/null/

But there's a dark side to being a Null, and you coders out there are way ahead of me on this. For those of you unwise in the ways of programming, the problem is that "null" is one of those famously "reserved" text strings in many programming languages. Making matters worse is that software programs frequently use "null" specifically to ensure that a data field is not empty, so it's often rejected as input in a web form.

Quotes and html blocks can contain payloads designed to inject code or database commands.

"The internet's one tough town"