It's easy to inadvertantly offend someone online via misinterpretation of tone, not thinking before you post, or just saying stuff you wouldn't say face to face.

And there's no reason why you shouldn't treat someone online with the same courtesy you would in real life. Yet we hide behind our veil of anonymity, chucking spears of ridicule with impunity.

So what's the most offensive thing you've said to someone online and then thought 'actually I'm a dick for saying that'?