Recent work by Alfred Mele, Romy Jaster and Chandra Sripada recognizes that abilities come in degrees of fallibility. The rough idea is that abilities are often not surefire. They are liable to fail. The more liable an ability is to fail, the more fallible it is. Fallibility is plausibly significant for addiction, responsibility, and normative theorizing. However, we lack an adequate account of what fallibility consists in. This article addresses that problem. Perhaps the most natural approach is to say (roughly) (...) the fallibility of your ability to F is the proportion of scenarios in which you do not F, among those in which you try to F. I argue that this approach (in all plausible versions) is mistaken. I then introduce the notion of an action’s “fragility,” and propose that we use that new notion to understand fallibility. (shrink)