Weinberg's "Rule of Three", which goes something like this: if you can't think of three ways a potential solution can fail, you don't completely understand the problem space.
Google found a couple other variations.
Jerry Weinberg says, “If you haven’t thought of three possibilities, you haven’t thought enough.” - quoted at stickyminds
and
Whenever I'm aware that I'm making an interpretation, I have another choice: I can allow myself to know that more than one interpretation is possible. A good check on premature interpretation is the Rule of Three Interpretations:
If I can't think of at least three different interpretations of what I received, I haven't thought enough about what it might mean.
This rule slows down the Interpretation step and gives me, the receiver, a chance to engage my brain before using my mouth. Even after I have thought of three possible interpretations, however, I should always be aware of one more possibility: that my list still may not include your intending meaning. -- Jerry Weinberg, Quality Software Management Vol 2, Chapter 6, quoted here
So, that's three varations on the Rule of Three...appropriate, no?
pensive