...is assuming the online statement is sarcastic when it's actually sincere.
"My mistake. I figured that he or she couldn't possibly be so ignorant|silly|dumb|unoriginal|hateful|presumptuous|arrogant."
It also has happened when I try to adjust to common interpersonal communication after passing time reading forums. People think I'm acting arch just because I look at them quizzically after they say something that's flatly ridiculous and I'm genuinely unsure whether to take them seriously.