I suppose possibly if you're travelling very late at night (as that's when things get reconciled)? However I'm not really an expert! Someone like
@MikeWh or
@Watershed might have a better idea! But either way it seems highly unlikely to happen in practice and particularly to OPs brother in law who will be travelling wholly legitimately!
As I understand it, the way that the system works is that barriers and inspection devices only store part of the full list of blocked cards. That part is refreshed/filtered so that it only includes the blocked card most recently used (or attempted to be used). I don't know what period that covers, but based on my experience of working with systems like this, I would hazard a guess at something between a month and a year.
So if your card was blocked a long time ago (e.g. because your account didn't have enough balance for the payments to go through for a day's travel) and you then try to use it again some months/years later, it might allow you to enter when you touch in at the start of your journey, since your card wouldn't be on the list of recently used blocked cards.
But before you complete your journey, the blocked card list would have updated to include your card, so if that flows through to barriers/validators quickly enough, you wouldn't be able to touch out and would have to speak to staff (possibly incurring an MG11 or Penalty Fare). It might also update quickly enough mid-journey so that an RPI doing onboard checks would see your card as blocked.
In my view this isn't really an acceptable solution since, when you get the green light on tapping in at the start of your journey, that is your authorisation to travel. It can't be revoked mid-journey in the way that TfL's systems basically purport to do. But we have seen a few cases where people have got caught out with that.
We have also seen the reverse issue crop up, where a card that was blocked for non-payment has been unblocked and re-authorised for travel (i.e. the overdue payment has gone through or been manually paid) and therefore someone is able to start their journey. However, they get stopped by an RPI part way through the journey because the RPI's device doesn't have the latest list of blocked devices (i.e. where the card is no longer showing as blocked).
