Server: WebProxy/1.0 Pre-Alpha errors? Are you tethering?

Today, I had to tether to my phone and run a few web requests top build a Raspberry Pi. However, the requests were failing. It would "sit there" and it didn't seem to work. I couldn't see anything wrong on the server, but I decided I would try a simple "curl -v" to see what would happen and to avoid TLS issues, I tried HTTP.

I was greeted with:

 < HTTP/1.1 503 Service Unavailable
 < Server: WebProxy/1.0 Pre-Alpha
 < Date: Tue, 23 Jul 2019 10:04:43 GMT
 < Content-Length: 0< Connection: keep-alive

What the heck? I knew my mobile provider had some proxies. I googled and noticed a few people complain about the same thing and then it occurred to me. I'm tethered, could this be my device? Would rebooting fix the issue?

So, since one of the troubleshooting tasks I'd suggest would be "Did you turn it off and back on" I decided to give it a whirl.

Guess what! That fixed the issue.