Roger Evans is a former member of the London Assembly.

It was a challenging audience. In late 2010 I addressed a gathering of media and film professionals at a special effects studio in London. They were worried about the future of the arts under the new Coalition government.

I saw plenty to be positive about as media and culture are big success stories which make a valuable contribution to the London economy. My speech seemed to be going well.

As I concluded, a man in combat fatigues stepped out of the crowd, levelled an automatic pistol at me and pulled the trigger. The noise was shockingly loud, my shirt exploded in a fountain of blood, and I pitched back onto the floor.

Everybody laughed and applauded. It was a great demonstration of special effects technology and the most original way I have ever ended a speech. The studio director was so proud that he asked if they could upload the video to Youtube.

I felt uncomfortable, so I refused permission – and just a few days later a madman ran amock with a gun, killing several innocent people.

No doubt the usual suspects would have accused me of causing offence. Journalists looking to exploit the story would have called Downing Street for comment. David Cameron, who never missed an opportunity to look tough by sacking more junior people, would probably have had my head on a plate.

Political instinct had helped me to dodge a bullet. I recalled this episode as a picture of Jeremy Corbyn visiting a train factory went viral over the weekend.

There are so many good pictures that could come out of this visit, but the one we saw featured the Leader of the Opposition standing in a toilet compartment wearing some odd looking safety attire. Cue many jokes about Corbyn going down the pan, seeking a new roll etc.

Labour supporters didn’t share the humour. They condemned the media for choosing the worst possible picture. But whose fault was it that the picture existed at all? An experienced politician and his media team should be able to spot pitfalls and take avoiding action.

Campaigning in Brixton with Steve Norris on his second mayoral bid, we spotted some large signs warning people not to use their phones in public. It was a graphic illustration of how Labour had lost control of street crime.

But Steve refused to be photographed anywhere near them: the strapline – “Lose It Here” – could have been so easily misinterpreted.

On another occasion I launched a strong report on bus driving standards in London whilst standing next to a double decker on London Bridge. Lingerie advertising on the side of the vehicle meant that the shoot had to be repeated with a different bus.

Good politicians need to consider every photo op with great care. You develop an instinct for the questions you need to ask. How might journalists make mischief with the content? How might the growing army of offence takers stir up a needless drama?

Misuse of material is not the fault of the politician or his advisors – but allowing it to exist in the first place certainly is.