This brings back memories, I was a guard at Guildford until five years ago and also a staff representative and I remember discussions with local management about platforms 6/7 at Guildford. The issue was that we only had one train that stopped on platform 6 back then and it was one of the last suburban trains out of Waterloo - a 2 x 455. The main problem as both sides saw it was where an Up Pompey came into Guildford and was diverted from the normal Platform 5 at the last minute into platform 6. The guard would be at the local door expecting to open the doors on the left hand side of the train. If the train ran into platform 6 the guard would still see a platform on the left hand side and, if they weren't careful, could mistakenly open the doors onto platform 7 in the belief it was platform 5. We agreed the “platform not in use” signs as a visual cue to guards, also many Guildford drivers would make a quick PA announcement saying we were going into platform 6 which was more for the guard's benefit than the passengers!
I personally opened the doors by mistake on platform 7, I was working a late suburban out of Waterloo at the end of a very gutty shift and when we ran into Guildford I had had enough for one day. I pinged the doors, picked up my bag, and heard a tapping noise from the opposite door; it was the shunter pointing and laughing at me from platform 6! I opened the doors on that side and stalked off in high dudgeon after making sure everyone had left the platform then reporting myself to Control; the controller's acknowledgement of my misdeed made it sound like he thought I was a numpty as well.
We didn't see opening the doors on platform 7 as a major safety issue however I have seen two passengers fall between a motionless train and the platform, as well as several others who slipped and almost fell down. The last suburban was nicknamed the 'Vomit Comet' especially Friday and Saturday nights due to inebriated passengers, many of whom would fall asleep and wake up at Guildford very disorientated, so the proximity of the juice rail on platform 7 was something of a worry.
In my case I had done everything right after my mistake, I think the local manager had a quick word with me and that was it. As TEW says above, as long as it is not a regular occurrence it's a pretty minor issue, least it was five years ago. These days the Guildford - Farnham shuttles regularly terminate and start from platform 6 at Guildford so I guess that local guards are now more aware of the platforms 7 issue at Guildford.