On Jan 29, 10:44*am, CJB wrote:
Recently I obtained a Hillingdon Community Services card which doubles
as a Library user's card. And put it into the same card wallet as my
Oyster card - in which I also keep my bank card. Suddently my Oyster
card stopped working on trains and buses, and even the Heathrow
Connect portable validators wouldn't recognise it - to considerable
embarrasment. The culprit was the Hillingdon Community Services card -
which seems to use the same technology as Oyster and was causing a
confict. An irritation 'cos now I have to keep them in separated. CJB.

A former colleague had similar trouble with his Oyster card and his
season ticket for a football club - Fulham, IIRC.
I think this problem's only going to become more prevalent over the
next few years.