Ask a freelancer about cash-flow problems and they'll tell you about late-paying clients. Real problem. But there's a quieter, more expensive one almost nobody tracks: the work you finished and never invoiced.
Source: [Dev.to](https://dev.to/giuseppe_tumino_8a3e54b7b/the-most-expensive-invoice-is-the-one-you-forgot-to-send-djb)