Whilst taveling around London today, I ended up at Tottenham Court Road
with what I assumed to be a Zone 1-4 PrePay ODTC on my Oyster, as well
as a remaining balance of 1.80GBP. Upon trying to touch in however, I
was confronted with an error. I checked my balance and found that I had
only 50p left. After adding 5GBP to the card I tried again.

I still got an error; checking the card again found no errors, and the
appropriate balance of 5.50GBP. I asked the guy behind the ticket
window about it and he said that my Oyster was clean. He told me that
there are occassionally propagation delays in the system and that I
should try again.

Trying again yielded the same error; I asked the guy manning the gate
to let me through, and after showing him that it didn't work, he let me
past.

When I finally touched out at Richmond, my balance was 6.80GBP!

What could have caused this rather unusual anomaly?