Lending someone money can be a slippery slope. You have to set out clear terms and conditions from the get-go, and have a plan in place for if it doesn't work out. That or you have to be okay with never seeing the money again. Because if it's between friends, things have a tendency to get complicated. You feel awkward asking for them to make good, and then it's on both of your minds whenever you see each other. This gets especially weedy if you're lending money to a coworker. You're already in a transactional relationship with them in some respect, so it's best not to blur the lines.
Not to mention that you have to see each other pretty regularly. There's no dodging a coworker unless you want to lose your job. That's why this story of a coworker pushing the limits of their professional relationship struck a chord with so many commenters. Read on for the wild thread.