It's "kind of" been suggested above, but essentially this wasn't a mistake, it was a deliberate calculation that it would be better to make a potential error and catch the train, than possibly miss the train and eliminate that particular risk of prosecution.
No matter how quickly that decision was made, or how much it relies on the OP's knowledge of the law or their temperament, the fact is that you do not accidentally decide not to bother checking your photocard, if you're unsure what order the number is in.
Before I worked on the railway I used to purchase weekly seasons, sometimes in a great hurry. Like many (most? I suppose) people, I had a ticket wallet into which I would insert my season, next to the photocard. I would be ready to insert the new ticket into the wallet after going through the barrier anyway, and knew I would need the photocard for the machine, therefore requiring a split second of planning on my walk to the station to swap the two actions around and ensure that I had in fact got the wallet, and thus correct photocard number, handy before I bought the ticket.
Now I do work on the railway, it is incredibly noticeable how people huff and puff when asked for their accompanying photocard, ask why I want to see it, or don't have it with them at all. Most people, however, are perfectly reasonable when the risk of high-value fraud is stated to them, because they don't want the railway to incur any more losses than it already does! The risk has been plainly stated here, too. Unfortunately there is every possibility that a station may have issued photocards with extremely similar numbers, so it is not a given that a ticket has been bought by or for someone else.