A careless comment posted on a social media network could land you in more trouble than you might expect, as Sally Bercow and a teenage Pink fan have discovered to their cost.

“Discrediting someone’s character, reputation or credit-worthiness is unlawful under the civil law and is known as defamation,” explains Martin de Ridder, a partner with Ansons Solicitors in Lichfield. “The court will not consider it a reasonable excuse that your actions took place late on a Friday night after a couple of drinks!”

A throwaway comment online could land the person who posted the comment with a substantial legal claim for compensation. For example, saying that a shop or restaurant has closed down when it is actually still trading, could cause a substantial loss of custom.

Sally Bercow’s tweet implied that tory peer Lord McAlpine was involved in the child sex abuse scandal in North Wales reported by BBC’s Newsnight. She was prosecuted when Lord McAlpine presented evidence that his reputation had been damaged by the number of people who had re-published her tweet and those likely to have read it.

Just this month, in Melbourne Australia, a teenager was detained by security ten minutes after arriving at a Pink concert after tweeting “@Pink I’m ready with my bomb..” in reference to her song Timebomb. As well as missing the concert, he had to face his parents who made a three hour round trip to collect him.

Also be careful what you publish about your working life. There have been numerous tribunal cases where an employer’s decision to dismiss an employee who made derogatory comments about customers or other employees has been upheld as fair. Whilst privacy settings can be used to restrict access to only your friends, comments shared by your friends can be visible to anyone. Some of your connections and activity on Facebook may attract the attention of your boss.

It should be no surprise that law enforcement bodies from the police to HMRC and local planning authorities monitor online activity, looking out for clues to any infringements. Evidence gathered from Facebook and twitter is increasingly being used in criminal cases and enforcement proceedings.

The safest rule for social media conduct is not derived from law but from the old adage, “if you have nothing nice to say, it is probably best to say nothing at all”.