When users are doing whatever they want

That's why math is so important. If the designer had known that where a^2 + b^2 = c^2 is true, |c| < |a| + |b| also holds true, he could have expected this.

@Fast-Nop nah, after a bit of user testing, the designer should have noticed that people were using the dirt instead of the road, and put armed guards at the two points where the dirt path and the road meet and made it a paid feature

Not dev but interesting:

There's a concept in design called desire paths. It's for situations like this. The places where people want to walk but you didn't build. Or conversely the places you built but they don't want to walk. It's why the grass dies, and why that "convienent" but needless sidewalk is never shoveled when it snows.

They usually only appear much later, but they make for great lessons in UX. People don't walk needless distance to make a clean 90 degree turn.

It's easier to relate to sidewalks and architecture, but I think it also applies to anything UX related if you're imaginative.