This sort of happened in a game I was playing in, ooh at least 15 years ago probably nearer 17 come to think of it! The opponents swapped keepers, in play without informing the ref, and with the new keeper midway through changing, with his gloves on and half wearing his new gk shirt and half in skins, he picked up the ball. Not really a save as such, just collecting another one of our many wasted long passes....!
I won't say yet what the ref did....!