We have a print-out of an N-set Venn diagram on the wall which we use to illustrate the peril of having too many features with subtle interactions. It resonates because many of us have worked on areas of the code preceded by comments indicating which subset of interacting features you need to keep in mind. There's at least 7 such features, so it's not a small space. We've been desperately struggling to make these features more orthogonal and having a succinct diagram of the problem (read: "enemy") has helped us stay motivated.