Inspired by meths' 'nice weather for ducks' thread: when people are all "look at this, look at this, it's HILARIOUS! I was pissing myself for hours!!!" And it's a YouTube video of a donkey lifted up off the ground because there's too much heavy stuff in its cart that even when you first saw it in about 2001 was only 'vaguely amusing'.
Or indeed when people use the phrase "nice weather for ducks, isn't it?" and look at you as if they think they've just invented a new paradigm in comedy (maybe doing that weird self-congratulating gurgle-laugh Chris Tarrant does whenever he speaks).
Personally unless it's an old person I feel sorry for, I refuse to do that little fake chuckle. You?