Took me a second to get what was going on here, but basically the idea is that you middleware might not see `C:D`, but then your application _does_ see `C:D`.

And given your application might assume your middleware does some form of access control (for example, `X-ActualUserForReal` being treated as an internal-only header), you could get around some access control stuff.

Not a bytes-alignment thing but a "header values disagreement" thing.

This is an issue if one part of your stack parses headers differently than another in general though, not limited to newlines.