Implicitly assuming that you mapped out/classified all possible realities. One of the symptoms is when someone writes "there are only two (or three or four...) possibilities/alternatives..." instead of "The most likely/only options I could think of are..." This does not always work even in math (e.g. the statement "a theorem can be either true or false" used to be thought of as self-evidently true), and it is even less reliable in a less rigorous setting.

In other words, there is always at least one more option than you have listed! (This statement itself is, of course, also subject to the same law of flawed classification.)