The project was profitable, on paper
Hours went in a spreadsheet, expenses in another, the invoice was written by hand. Nobody can say which project actually paid.
Retainers are consumed by memory
A client bought a monthly bundle. Whether it is used up by the 20th is something a person has to remember, not something the system says.
The same hour is entered twice
Once in a timesheet, once in the invoice. Between them sits a person and a chance to get it wrong.
Every country adds a system
A subsidiary in another currency, another VAT regime, another chart of accounts. The consolidated picture arrives late, if at all.