All the time!
I usually start typing something, and... (I'm already tempted to do it now)... and realise that it's not translating so well into words, sounds poorly thought-out and inarticulate; I often don't post out of fear of looking stupid. We are all anonymous obviously, but I still care very much about whether other users here like what I've got to say.
Even when I first went into chat I was a little terrified.
But to be honest, I mostly don't post because I have very little free time to write out well worded responses; I don't like to post things unless it's well thought out and worded very carefully (...and heavily edited).
With the exception of this post, which I've more or less just banged out willy nilly, pell mell and without shame.
I am a piece of knowledge-retaining computer code imitating an imaginary organic being.