Until I was seventeen, I thought that everybody had the same difficulties noticing body language, and that everybody had to grapple with the same problems with socialising as I had to. I thought, basically, that all other people had faced the same difficulties as I and that, since they all seemed to have succeeded were I was failing, I was perhaps not trying hard enough.
(I'm noticing now that my answer is pretty much the opposite of what you actually asked for, since it's a time I thought people didn't know something when in fact they did, so I'll try again - forgive me leaving the initial answer in)
I tend to make the assumption that, if I know something, then everyone knows it. Just today I was asked who the person in my avatar is, and at first my reply was just, "Blind Willie McTell." It wasn't until later that I remembered that few know who that is, so I went and edited my response so that I explained who he actually was.
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How long did you lurk on these forums before you signed up and started posting.