...in a series of studies, participants were only able to accurately communicate sarcasm and humor in barely half—56 percent—of the emails they sent. What's worse, most people had no idea that they weren't making themselves understood.
In my experience, people tend to think I am being humorous when I am being serious, and serious when I am being humorous. Of course, I view that to be user defects, and not a defect of my writing.
via Guy Kawasaki