Flow network and residual network

So I'm studying for a test in Algorithms and can't figure out the trick in this question:

I need to show an example for a flow network with the flow f, in a way that in the residual network there is a path between s (source) and t with capacity bigger than 0 that makes the flow go on an edge that do not exist in the original flow network. I need to explain how to increase the flow in the original network.

So If this edge do not exist in the original flow network, that means his capacity is 0. How Can I still make the flow better? since in the residual it's only flow that "go back" in the network. I thought maybe since the flow is going back, I can use in for other path?

The path: 1->3, 3->2, 2->4