I'm not superstitious, but when I'm walking on sidewalks or tile floors I have tended to try to avoid cracks or lines. It's an easy but engaging puzzle to try to do it while maintaining a normal gait, like the ambulatory equivalent of Sudoku.

Then, one day, my high school geometry teacher taught us about angle bisectors and the game changed permanently. Now, in addition to visible lines, any line intersections now produce invisible bisector rays that must also be avoided. I made a picture to show where you can't step on a sidewalk. It has been decades since high school geometry and I still try to avoid bisector lines any time I'm on a suitable floor. I have never added another rule to the game since, and it wasn't til this post that I thought about how strange that is.