vertaaltnu Very interesting question, @vertaaltnu! Just like @eleonorang above I was quite lucky so far and I never had to return the job for unforeseen reasons.
I guess you can't be ready for unforeseen things such as illness, accidents or forces of nature. What you can do, however, is to plan ahead and make sure you don't bite more than you can chew.
This is exactly why I refuse to do urgent jobs with unrealistic deadlines. I always prefer to be on a safe side and whenever I propose a deadline I often give myself some sort of a buffer in case there's an emergency and I don't have access to my computer.