Code doesn't lie, but it isn't always obvious either. A diff that fixes a subtle corner case is very difficult to understand without explanation. A function to

Code doesn't lie, but it isn't always obvious either. A diff that fixes a subtle corner case is very difficult to understand without explanation. A function to fix dirty external data cannot be understood without reference to what the author is fixing. It may not be clear why a certain performance trade-off is preferable. A mistake may be hard to detect if you don't know what was intended.