"Facebook apologises to drag queens and transgender people after deleting profiles" is a surprising headline, in the technical sense of describing a set of events that would ex ante have attracted a relatively low probability. However, the chances are it's not something anyone, including apparently Facebook's policy people, gave very much thought to. Facebook was enforcing its 'real names only' policy by deleting clearly-pseudonymous profiles. Apparently pseudonymous profiles are a mode de vie for many drag queens. Who knew? (Answer: the transgender community.)
In this regard, it's a bit like The Arab Spring, Russia's annexing of Crimea, or The Only Way is Essex. These were all things that weren't so much dismissed as low probability, so much as barely considered as possible outcomes. This is a category of error now commonly known as 'failure of imagination'. Unlike the various probabilistic reasoning biases, which are well-understood and relatively easily corrected for, there isn't a set of steps you can follow to guarantee immunity from failure of imagination. The evidence concerning structured brainstorming, which is widely regarded as one of the easiest and most effective collaborative imaginative techniques, only suggests a limited impact on the productivity of ideas compared to other approaches.
Importantly, however, failure of imagination is sometimes a risk it's optimal to take. Generating hypotheses and scenarios has a cognitive and sometimes economic cost. For relatively easily-rectified decisions (such as Facebook's) it's not usually worth spending too much time thinking of all the outcomes associated with them. By definition, surprising things don't happen very often. In practice, we should always assume that alongside the things we have thought about, there is a small probability of some important thing that we haven't, and plan accordingly.