Malcolm & Nika wrote to uk.transport.london on Thu, 25 Nov 2004:

Its the magnetic strip getting wiped by something. Common cause is a
magnetic clip on a wallet of handbag where it is kept. Another common one is
the magnetic bracelets the older folk tend to wear for joint relief etc. Can
also be done by mobile phone in close proximity. Happens very regular.
Cant it now be done at your local station. It was introduced like that about
2 years ago but it may have changed perhaps. Just ask the nice young lady on
the barrier to see if it can be done locally.

I understood the OP was talking about an Oyster-like card, not the kind
with the magnetic strip. Those die very easily indeed!
--
"Mrs Redboots"
http://www.amsmyth.demon.co.uk/
Website updated 22 November 2004