One small business I have seen just has tonnes of word documents sitting in a single folder, with a mix of customer details, invoices, etc. To write a new invoice they copy an old one, then change the details, save it with a new name with a "system" (that changed over the years).
Up until semi-recently I saw a small-medium sized business (hired quite a few people) that did all of their accounting _by hand_, for hundreds to thousands of transactions. They had an old lady that came in to manually run their accounts on a calculator and it took several days.
A small business I once dealt with had a small black book (A6?) that was all written in note form, scattered, not dated, random important scribbles everywhere, mix of cash and card (not documented) - it was a nightmare. Somehow they had a good reputation in the local area, but behind the scenes it was pure chaos.