On Mon, 11 May 2009 12:22:16 +0000 (UTC), Tim Woodall wrote:

I've also had a case where it has opened the gates for me but beeped
ominously and said "seek assistance"[1]. (Can't remember where but it
was a station I'd never used before). When I asked at the window they
said "Don't worry, it does that sometimes" but they did check my journey
history there and then and the exit had registered properly so maybe it
was something I did...

I see this happen to people all the time. I think it happens when you
hold the card against the reader for slightly longer than necessary -
it reads the card and opens the gate, then attempts to read the card a
second time, and this time rejects the transaction as a duplicate.