Last Friday, being ultra-organised, I purchased a Zone 1-2 weekend
travelcard from Blackfriars station at 2100 Friday evening - knowing
that I would be getting the tube home after midnight, and then using
it all weekend...

Upon trying to use the ticket at Waterloo at about 0015, I got the
'seek assistance' message and the station assistant told me it wasn't
valid until the first tubes next morning. Not wanting to debate the
issue (else I miss my last tube), I bought a single ticket.

I tried to use my ticket on Saturday morning, and once again got the
'seek assistance' message. I went to exchange the ticket for one that
worked - and the assistant told be that it didn't work because I'd
tried to use it before it was valid. When I told him this was after
midnight and that I thought it was valid at that time, he said 'yeah I
thought so too'.

It seems clear from TFL's ticketing information that it should be
valid from 0001 on the day of validity? If so, why did the ticket
barriers not accept this? Anyone else been caught out by this?