Losing your grip on the real world: I’ve done this myself, when you forget that what you (and maybe your social circle) know and understand does not necessarily mean that everyone knows this, but not with such persistence as an old friend of Andrew, a former boss of mine. When they were both mathematics students at Cambridge it became clear in conversation that this man assumed that everyone knew at least the fundamentals of tensor calculus. Andrew told him that hardly anyone had even heard of it, but he wouldn’t believe this. Andrew took him to the local supermarket and asked the checkout girl, who apparently looked at him as if he was a lunatic. The friend still wouldn’t believe it, and maintained for years that Andrew had clearly planned all this and bribed the checkout girl to pretend that tensor calculus was unknown to her.