Debugging is twice as hard as writing the code, so code written as cleverly as possible cannot be debugged by its author.
Kernighan's corollary is the operative part: if you write code at the limit of your cleverness, you are by definition not clever enough to debug it.
In practice
- An engineer writes a one-line expression that handles four cases and cannot explain it to themselves six weeks later at 3am.
- A finance model built with elegant nested formulas cannot be checked by the person who built it once the numbers look wrong.