John Clausen wrote:
On Saturday I travelled around London on tubes and National Rail
using my Oyster card instead of a paper travelcard. My advice to
others thinking of doing the same is: don't bother. Instead of
capping at £7.00 it kept charging me until there was no credit left,
over £17 in total. Some credit has been put back but I have still
been charged £14.60.
There were three of us travelling together and we all got print outs
at Victoria in the afternoon. Various parts of the journey were
ignored including a trip to Harrow on the Hill where we touched out
and back in again at Marylebone and the same at Harrow. So the system
thought we had made a journey from Paddington to Vauxhall taking 95
minutes which is over the time limit. The bloke at Victoria suggested
that we hadn't been touching in and out properly but then how would
the gates have opened? The fact that three of us had the same problem
suggests that it is system failure rather than user error. Has anyone
else had similar problems? I will not be using Oyster as a travelcard
again in the near future.

The system just isn't designed for people travelling around the way 'rail
enthusiasts' seem to. This has been widely discussed in uk.transport.london
for the last few years, in fact if you check there this post has just been
cross referred in the latest discussion, about the best way of travelling
around as a tourist for a few days!

Touching out and touching straight back in at the same station doesn't
necessarily end and restart a new journey, and there are logical reasons for
this. Once a journey that has been joined to another goes beyond the time
limit (and that limit is variable dependent on zones crossed) the first part
of the journey becomes 'uncompleted' and the latter part 'unstarted'. This
is charged as two 'maximum cash fares' and doesn't contribute to the daily
cap.

I've had this happen a couple of times and it has been sorted with a call to
the help desk. What they can do is sort out the 'unresolved' journeys into
consecutive single journeys, then reapply the daily capping properly.

[uk.transport.london added for more expert explanations]

Paul S