Unfortunately, though, the man's employers failed to see the funny side, and promptly fired him. The same thing happened to the Buckingham Palace guard who called Kate Middleton a 'stupid stuck-up cow'.
A surprisingly common mistake is to forget that you're Facebook friends with the boss - like the woman who was fired after describing her manager as 'pervy' in a post he saw just hours later. More usually, though, it's a colleague that puts the boot in. And it's not just personal remarks that can get you in trouble; it can be anything that could cause your company to be seen in an unflattering light - like the Virgin Atlantic cabin crew who described seeing cockroaches on their planes.
"Judges have held in several cases now that anything you put on social media is public," says employment specialist Michael Scutt of Excello Law. "Case law seems to be developing that if it does bring an employer into disrepute, even if you don't name the company, as long as people can work it out - because the employee's friends know where they work - then that is going to be enough."
"If employers do start trawling social media accounts and discover an employee is gay, for example, but they're discreet about it at work; well, if that person is then made redundant, even if the motivation wasn't homophobia, it does give the chance for the employee to complain," he says.
"It also depends on how the employee reacts; if they agree to take the post down and be very, very contrite, then I think there's a strong chance a tribunal would consider that it was unfair dismissal if they were fired."
Increasingly, though, the problems can begin before you've even got the job. A survey last year by PR agencies Eurocom Worldwide and Six Degrees found that a staggering 40 percent of technology companies look at job applicants' profiles on social media sites before offering them a position. Indeed, Six Degrees' managing director, Jennifer Janson, says she does it herself.
"We certainly as a business owner when interviewing candidates will look at their social profile," she says. "It can add colour to a CV and be a very positive thing. I don't think I'd ever reject anybody on the basis of their social media profile, though."
However, plenty of people do - one in five, according to the survey. "Why wouldn't I?" asks one manager with an IT firm. "We get hundreds of applications for every job, and it's a good way of weeding people out. If their Facebook page is full of stupid comments or pictures of them getting roaring drunk, then they're probably too dumb for the job."
A recent study by On Device Research found that nearly one in ten young British people believes that they've been turned down for a job because of their social media profile. While this may be wishful thinking - there could be more compelling reasons for their rejection, after all - it does show that job applicants are at least aware of the dangers. Unfortunately, though, it doesn't appear that they're altering their behaviour accordingly, with two-thirds saying they don't think their use of social media now could harm their future career prospects.
"If getting a job wasn't hard enough in this tough economic climate, young people are getting rejected from employment because of their social media profiles - and they don't seem to be concerned about it," says Siim Teller, On Device Research's marketing manager.
There have been moves - strongly opposed by Google in particular - to try and give EU citizens the 'right to be forgotten' and allow them to demand that embarrassing information be taken down. Unfortunately, a ruling this summer established that this right does not exist under current law.
So how to prevent your social media past from coming back to haunt you? You can, of course, simply close your accounts down. Otherwise, it's a question of using privacy settings to the full, while being aware of their limitations.
Many people, for example, fail to realise that on Twitter, all tweets are completely public by default. On Facebook, meanwhile, limiting posts to Friends Only stops strangers seeing what you got up to last night. But some basic information is always publicly available, such as your profile picture and 'networks' such as your school and university - so think very carefully about what you choose to include.