If it is impossible, it did not happen

If a bug can only be explained by an impossible event, then - almost always - your assumptions of what happened are wrong, and once in a blue moon, your understanding of the possible is incomplete.

I was working on a section of code in a large, old, and tangled code base. I passed a method three arguments, but the method only admitted to receiving two. The code looked something like this:

function checkPurchase(item, price, budget) {
    console.log(item + ' - ' + price + ' - ' + budget + ' - ' + arguments.length);
    ...
}

...
checkPurchase("item", "price", "budget");

The above always showed:

item - price - undefined - 2

This looks like some sort of horrible core JavaScript bug where arguments are being lost. Clearly this is approaching the level of impossible. After hunting around for a while, I realized the method was being spied on and called through, but only the first two of the arguments were being passed on, since the legacy code only had two arguments….. the spy had been baked into the code for some quick testing in an unrelated file and forgotten about…. so somewhere else in the codebase there was a:

var oldCheckPurchase = checkPurchase;
checkPurchase = function(item, price) {
    // Do some checks etc here
    ...
    oldCheckPurchase(item, price);
}

So if the only possible solution for your bug is something close to impossible - like JavaScript randomly dropping the third argument of one specific method - then you probably failed to consider something much more likely.