Fare evasion dilemma

Any legal eagles amongst you?

Back in London now and thanks to Neil for holding the fort while I was in sunnier climes.

While I was away, someone left a message on goingunderground's message board with the following question:

"HELP! - A few weeks ago I was stopped by a plain clothes officer, and as I did not have a valid ticket, so I was told I would either be cautioned or prosecuted. I have received a letter saying that I should write to them explaining my case so they will decide whether to prosecute or caution me. The thing is, in spite of having a valid ticket at the time, I touched the wrong one on the machine (which belonged to someone else) by mistake. I was advised that I can only get a caution if I plead guilty, but in this case they might still prosecute. Does that mean that if I tell the truth, that I touched the wrong ticket by mistake, I will definitely be prosecuted and have criminal record for £1.50??? I really don't know what to do and would appreciate some advice. Cheers!"

So what happens in situations where you do have two tickets and you use the wrong one by mistake? There must be genuine cases where this happens. Or would TfL see this as the fare evasion excuse number 239, in the great collection of Lost Travelcard Excuses? Surely if you explain your mistake they would understand - because technically you have actually paid your fare and are travelling with a valid ticket - but just not showing it.