Halloween: the scariest bugs make no noise

Every horror movie knows the loud scare is the cheap one. The thing that actually gets under your skin is the quiet house, the door that's open a little wider than you left it.

Bugs work the same way. A 500 error is loud. It pages someone, it shows up in the logs, it gets fixed by lunch. The ones that should scare you return 200 OK, render nicely, and are wrong.

Here are four that haunt us.

The link that leads home

A customer clicks a product link from an old email. The product was removed last month. Instead of a 404, the site answers with a 302 and drops them on the homepage.

Nothing is broken, technically. No error, no log line worth reading. The customer just wonders where the thing they wanted went, and search engines slowly learn that a pile of your URLs all mean "homepage." This is why a silent 302 is one of our universal rules, checked on every step of every run.

The free lunch

An in-stock product, ready to add to the cart, priced at 0.00.

Maybe it's a sync job that ran half-way. Maybe a discount went negative and got clamped. Either way, the page looks finished. In our first dry run on a partner's staging site we found ten of them. Nobody had noticed, because nothing about a 0.00 price looks like an error to a computer that isn't told otherwise.

The sort that lies a little

"Lowest price first." The list looks sorted. Mostly it is. But it's sorting by the original price, not the discounted one the customer sees, so a cheaper item sits a few rows down.

No one files a bug about this. They just scroll, get a vague feeling the site is off, and buy somewhere else. We saw this one in the October dry run too.

The error in the dark

A JavaScript error fires on page load. The page still renders. The banner still spins. Nobody opens the console, so nobody sees it.

Until the error is the reason a filter doesn't apply on one browser, or an analytics event never fires, and someone spends a week wondering why conversions dipped. Ours, in October, was a homepage banner initialized twice.

Turning on the lights

The trick with silent bugs is that you can't wait for them to make noise. You have to decide in advance what "wrong" looks like and check for it every time: no silent redirects, no in-stock product at zero, sort order matching the displayed price, no console errors. Boring rules, written once, running forever.

That's not very spooky. It's just how you stop being scared of the quiet.

Happy Halloween. Check your console before you go trick-or-treating.