As reported by an anonymous contributor on AccountingWeb, using the pseudonym 'Welsh dragon':
We took over a client a few years ago. The reason he had decided to move was that on Christmas eve he had telephoned his accountant and said to him that he and a few colleagues were in the pub next to the accountant's offices and would the accountant like to join them. The accountant said "no".
Next time the client received a bill from the accountant he noticed that he had been charged for the accountant taking that 'phone call.
That single entry cost the accountant a client - and no, it wasn't the well known firm of Scrooge & Marley.