Oyster pads on ticket gates ”know” which direction they’re facing, so if you do this (unless you have a Travelcard valid at that station) you’d get a maximum journey charge for touching out with no corresponding touch in, and a second one upon touching out at your eventual destination.At some stations I'm curious as to whether it might work to reach over and tap my Oyster on the other side of the barrier and walk through a ticket barrier 'backwards', or whether it'd be counted as a tap out (I intend to pay, but not perform some sort of bizarre dance to get to the correct side of a platform)
You’d also be committing an offence under the Railway or TfL Byelaws (whichever it is that applies) by passing through a ticket barrier otherwise than in the correct manner.