I company I used to re-wire most of a house (cost was about £5k) in total but spread over several invoices sent me another invoice for about £550.
When I asked for an itemised invoice, I got 3 different invoices - one of £300 which I had already paid.
It's taken me about a month to figure that out because, a) I'm a bit flaky 
b) I'm really busy and this is a side project that I have to do, but I've much more important things going on at the moment c) I've been away for 2 of those weeks on holiday & d) I work away from home and don't always have time/ or my paperwork in front of me.
I emailed the company to check their records because I was going through my bank statements and I had paid the £300 as requested several months ago on 10th March
I got this reply....
Good Afternoon
I have allocated the payment from 100thh Marchh_.
I am attaching a statement showing the remaining balance of £255.
Kind regards
I'm a bit of a gob-shite so trying not to write a passive aggressive, 'no need to apologise..we all make mistakes' but AIBU to expect a 'sorry for the confusion, you are right x invoice was paid on 10th March, our apologises'. Or am I expecting too much? Genuinely would like to know if this would annoy you?