An example I use to talk about hidden edge cases:

Imagine we have this (pseudo)code

  fn doSomething(num : int) {
    if num % 2 == 0 {
      return  Math.sqrt(num)
    } else {
       return Math.pow(num, 2)
    }

  }

    assert(doSomething(4) == 2)
    assert(doSomething(3) == 9)

Except there’s a bug in this; what if you pass in a negative even number?

Depending on the language, you will either get an exception or maybe a complex answer (which not usually something you want). The solution in this particular case would be to add a conditional, or more simply just make the type an unsigned integer.

Obviously this is just a dumb example, and most people here could pick this up pretty quick, but my point is that sometimes bugs can hide even when you do (what feels like) thorough testing.

Prior to generative AI I was (correctly) criticized once for making a 2,000 line PR, and I was told to break it up, which I did, but I think thousand-line PRs are going to be the new normal soon enough.