Often when trying to stifle a cough, or avoid choking, I make sudden snorting noises that sound like I am stifling a laugh. It's annoying when someone who's not saying anything remotely worth mockery thinks I am snorting with derision, and responds to my 'laughter.'
person: "After she was tortured..."
person: "Heh heh! Yeah, I know."
Even worse is when they think I think they are Robin Williams, and keep making more jokes.
Far worse is when they know I don't think they are funny, but they think they are Robin Williams, and keep making more jokes.