I make many social mistakes online,
e.g., I tend to indulge my personal sense of humour but fail to anticipate that others may take it as a serious expression of how I feel
I can sometimes respond inappropriately to other people because I've failed to understand their position on a subject (which usually leads to prolonged bickering matches) -- sometimes this is because I'm tired, sometimes it's because I'm drunk, and sometimes its because I simply wasn't paying sufficient attention
While I try to behave in as empathetic a manner as possible, I sometimes badly misjudge a situation and post seriously non-appropriate remarks, often because I don't appreciate the extremely personal and sensitive nature of the post to which I am responding, for whatever reason (although in my defence I will say that these qualities are not always immediately obvious without having read the whole thread and considering the individual character of each participating poster -- and who can honestly say that they always bother with all that?)
I sometimes have strong opinions and while I try to present these in an easily-comprehendible format, the combination of strong opinion and simple language can sometimes come across as quite blunt, or even as aggressive, which always amazes me when people react to it on that basis, because I am simply not an aggressive person at all
I am far to ready to enter into debate at times, and therefore appear to be picking fights with people -- I am aware of this tendency and am making ongoing efforts to ensure that my desire for active debate is expressed and pursued in more reasonable tones (but at root, I just love a good old cut-and-thrust take-no-prisoners debate, clear outcomes involving obvious winners, obvious losers, and deserved disqualifications)
I am broadly left-wing in my political outlook (with a particular fondness for 'Old Labour') and will often perceive any expression of right-wing views to be tantamount to a declaration of ideological war (when I know in my heart that I should be most productively employed in seeking common ground and shared social values with right-wingers -- but by Christ, it's hard to maintain that outlook when most of them are self-evidently such utterly amoral, selfish, socially-irresponsible shitbags)
Last but by no means least, I have recently tended to post while under the influence of recreational stimulants, which has led to me frequently making over-long, extravagantly-detailed, unnecessarily-complex, highly digressive and perhaps ultimately unreadable posts, which (while they might prove to be perfectly sound in their logic, and reasonable in their overall character, if tackled by a determined reader) can generally be summed up as TL;DR. I will endeavour to make fewer of these megaposts in the future, and aim to encapsulate my arguments more succinctly.
That's all I can think of for now, although I'm sure there are other forms of social mistakes that I make on line (I was recently described by another WP user as 'annoying', which is probably a quality beyond my control, so I'm sorry if this is the case)