Platform 4 at Finsbury Park. It exists, but only as a mirror of Platform 5. Since they both serve the up slow, the doors only open on the left-hand side of the train.
This example is true of quite a few stations, such as Ascot. In this instance though, I am not sure which platform is the dud!
Ditto for Guildford platforms 6 & 7. Both face onto the same road, but trains with powered doors (I.E: All rolling stock nowadays) only open on the plat six side.
Nowadays, seven's pretty much just extra space for plat. 8...But it was
really useful back when we had CIGs working down Guildford way! When I was using Guildford regularly, my train (The FGW to Blackwater) had a tendancy to have its platform altered at the last minute.
It was always quite hillarious watching everyone make a mad dash from one platform to another via the bridge or subway...Whilst I simply boarded the CIG standing at 6/7, opened the door on the other side, closed the door through which I'd first passed*, then stepped out onto the other platform!


* - The reason for my open door A, open door B, close door A, close door B approach was a simple guard to ensure that the train didn't depart on an ECS move etc. whilst I was using it as a bridge!