Credit card charges

[LordGrover]

I'm pretty good at paying my bills so was a little surprised to see a 'late payment' fee of £12 on my statement. To further rile me, it shows an interest charge of £136.27 too.
Payment is due on 29 of month - it left my account on 24 December but didn't show up on credit card statement until 30 December.
When I phoned to query it I was told by a very nice person that they were very sorry but had quite a backlog over the holiday period and a lot of payments weren't reconciled on time. Not to worry, they'd refund my account before the next statement!
WTF? If I'd not checked my statement they'd have had me for £148.27 because their systems screwed up.
How many others have been charged and not noticed it?