Every year, at the first faculty meeting, representatives of the registrar tell us what percentage of the incoming class is [insert variable in which we are interested, such as American Indian, working class, international, etc]. They compare it to last year’s percentage. This drives me crazy because they do so as if comparing the last two data points in a sequence is indicative of a trend. But to determine whether or not there is a trend, and therefore whether the increase or decrease in the percentage of [insert variable in which we are interested] significant relative to last year, depends on more than two data points!
xkcd does an excellent job of illustrating just how two data points can be utterly meaningless, even wildly fallacious: