docs: add debug tip (ory#2186)

docs/docs/debug/troubleshooting.md

@@ -12,6 +12,16 @@ either open a [discussion](https://github.com/ory/kratos/discussions) and ping
 
 :::
 
+### `400: Bad Request` on self-service flows
+
+Make sure you are starting and finishing the request in one browser.
+Self-service browser flows need to be executed in the same browser from start to
+finish!  
+Starting the flow in e.g. Safari and completing it in Chrome won't work. API
+Clients like Electron, Postman or Insomnia are browsers themselves, which can
+cause requests to fail. For testing purposes [cURL](https://curl.se/) is a good
+choice.
+
 ### How can I separate customers/employee data, but have them use the same login dialog?
 
 > We want to separate our customers and employees, so we store them in different