docs: document missing headers problem

website/docs/guides/troubleshooting.md

@@ -72,3 +72,21 @@ If you don't consume this publication, the WAL can fill up and your Postgres dat
 ##### Solution — run Electric
 
 The simplest way to avoid this is to make sure you're running the Electric sync service against Postgres. This will consume the publication and allow the WAL to be released.
+
+### Missing headers — why is the client complaining about missing headers?
+
+When Electric responds to shape requests it includes headers in the response that the client needs in order to function. It's common to run Electric behind a proxy. However, some proxies might not pass through the response headers. In this case, the client will complain about missing headers.
+
+##### Solution — pass through the response headers
+
+Make sure that the headers in the response from Electric are passed-through to / copied onto the response that the proxy sends to the client. It's safest to pass-through / copy all the headers. At a minimum, you **must** pass through any headers prefixed with `electric-`.
+
+### CORS headers — why are cross-origin requests not working?
+
+Electric also exposes CORS headers. In some cases if a proxy overrides `Access-Control-Expose-Headers` when it responds to an `OPTIONS` request, this may break CORS.
+
+##### Solution — pass through CORS requests and response headers
+
+The safest thing is to also always make sure you proxy `OPTIONS` requests to Electric and pass through any CORS headers in the response from Electric to the client as well.
+
+If in doubt, pass the headers through!