Yep, thanks! So not middleboxes or CAs, but old rusty servers.

The other example of evading Hyrum's Law that comes to mind was when early Javascript users of JSON observed how they could intersperse comments.

Crocker said he noticed people were using the comments to stuff preprocessing directives into JSON.

He then devised the most ingenious hack: He told people they weren't allowed to put comments in JSON. Then people stopped putting comments in JSON.

I'm starting to wonder whether Hyrum's Law is really more of a suggestion. :)