Got on the bus on Saturday, and concentrating on the stupid tiny little
screen so that I should see my remaining balance I didn't notice whether I
got a red/green light (or a beep)

When nothing came up on the screen I asked the driver if it had registered,
and she said no.

So I "tapped in" again and got "card already used for this journey", driver
looked bemused, I shrugged and sat down.

And now on obtaining a printout of my journey history I find that I didn't
make a registered bus journey at 18:00 on Saturday.

So how did that happen (and I dread to think what the conversation would
have been if an inspector got on - he wouldn't have believed me, would he!)?