I once had an experience where I was 100% certain there was a technical glitch with the Oyster system, and their customer services ended up agreeing with me too. I was making a journey from Turkey Street to Chafford Hundred via Seven Sisters/South Tottenham and Barking, with the following touches:
In: Turkey Street
Out: Seven Sisters
(OSI)
In: South Tottenham
Out: Chafford Hundred
There were no staff in the western concourse at Seven Sisters, and the manual gate was open too, so rather than going through the automatic ticket barriers I passed through the manual gate and used the standalone validator next to that gate (not the ones in the passage from the concourse to the NR trains, which so many people seem to mistakenly use, although I know the Oyster system can correct for that). Bizarrely, the standalone validator thought I was entering the system, not exiting. Which, in turn, meant that the validator at South Tottenham thought I was making an exit, resulting in me effectively travelling ticketless from South Tottenham to Chafford Hundred, though no fault of my own. I'm glad I didn't bump in to a ticket inspector!
When discussing it with someone at the Oyster help desk a few days later it was deemed that I had touched in/out correctly, so the Oyster person was baffled as to how/why it happened. Nevertheless, since then I've made a point of always choosing gates rather than a manual validator, as gates know for certain whether you're entering or exiting so they can't make that same mistake.
The point of this post, anyway, is to say that despite the fact most apparent glitches with the Oyster system can be explained as being the result of intended but unusual circumstances (e.g. as with the Finsbury Park continuation exit issue discussed above), occasionally the Oyster system does throw up a genuine technical glitch.