API [Authentication] - invalid_client

When going through the standard OAuth2 flow, "invalid_client" is returned after the user clicks "Grant".

Environment

Custom Applications with users going through the standard OAuth2 flow.

Steps To Resolve

1. Make sure the client_id is properly spelled when the user is first directed to the OAuth2 URL. It's not uncommon to miss a letter or number at the end.
2. Pipe parameters (e.g. "|") in the redirect_uri are not supported and will also generate this message.

Root Cause

The client_id in the OAuth2 URL must be a valid client ID, and the rest of the redirect_uri must be properly formatted.