Why I Never Make Mistakes. . . Except When I Do

a Gurkha soldier on patrol in what is clearly Lancastarian Street, Carrickfergus

A few weeks ago on twitter someone pointed out to me a mistake in Gun Street Girl...Apparently Sean Duffy drives down the M40 motorway to Oxford a couple of years before the M40 was actually finished and when in fact it was still a dual carriageway. I thanked the person on twitter for their close reading of the text and their observational skills. I was worried that the mistake might have spoiled some of the enjoyment of the story so I then explained to my twitter interlocutor that one way of looking at it was to consider Sean Duffy's fictional universe as more or less identical to our universe but with a few subtle differences: among these differences were the somewhat extraordinary events of Gun Street Girl itself and the fact that M40 motorway was finished several years earlier in Duffy's universe than it got finished in ours. My twitter correspondent was satisfied with this. 
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I have played this hand before. A couple of years ago someone emailed me to complain that the Gurkha regiment was never posted to Northern Ireland yet (entirely for the purposes of a joke) I had Sean Duffy stopped at a checkpoint manned by Nepalese soldiers. I explained that Sean Duffy's universe was almost identical to our universe but one of the differences was that in 1982 Gurkha regiment soldiers were indeed sent to Northern Ireland. And at a reading in Scottsdale once an aggressive man came up to me to explain that I had made a mistake in one of my Michael Forsythe novels: "The Glock 17 you gave him in Bloomsday Dead does NOT have a safety on it. You wrote that it had a safety! Everybody knows Glocks dont have safeties!" he said triumphantly. I am not a firearms expert by any stretch of the imagination but I was not non plussed. "No, sir, that's not a mistake," I said. "Michael Forsythe inhabits a fictional universe that is nearly identical to our own universe but one of those differences is the fact that in his universe all Glock 17's come with safeties." The man's look of triumph changed to one of mild irritation.* If you are a novelist or short story writer please feel free to use this excuse free of charge. But I should warn you that it doesn't always work. Also on twitter a few weeks ago someone pointed out that in one of the Duffy novels he's driving his trusty BMW but later on in the same chapter it becomes a Land Rover. Now this is what I call a mistake! (A copy error that slipped into the first printing and which now has been mercifully fixed for the mass market pbk.) By no species of logic can this possibly work. And that's the key isn't it? If a book is not consistent by its own rules of physics and logic then the author needs to own up and admit it. I did and hung my head in shame. 
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*Later I found out that the guy was talking shite anyway. Nearly all Glocks come with a variety of safety features...