The craziest bug I have ever seen

Imagine if someone came to you and told you that your website was causing their laptop to throw a fatal system error, the dreaded ‘blue screen of death‘, what would your response be?

Well I know what my response would be because it happened to me. My response was “no way! That can’t happen! A website can’t make a computer BSOD!” I would have bet $1000 on that. Turns out I was wrong; very wrong.

I was working for a very popular pizza delivery chain and one day our development team began receiving reports of customer’s complaints (mostly via social media) that our site was causing their computers to throw blue screen of death errors! We laughed about it, yeah right, that can’t happen! Crash a browser tab maybe, but not an operating system. But we tried to reproduce it anyway on the large number of laptops we had and no matter how hard we tried we couldn’t reproduce it.

A few days later a member of our customer support team appeared saying our site just BSOD’d his laptop! We were curious, very curious. So we started the laptop back up and visited our site and voila! BSOD!

Now here’s where you might not believe me, so I took a video of it as proof, for your enjoyment:

Now that we had a single laptop that consistently reproduced the BSOD we could work out why it was happening.

It was only happening in Chrome, and only on this single laptop which ran Windows 8. We built/ran our site on a developer’s machine and accessed it via this laptop and could reproduce the crash every time.

We noted that the version of Chrome was one version behind the latest version on every other laptop we had – the update had somehow stopped and was stuck at that version.

But we didn’t update Chrome as that would have destroyed our single machine that was reproducing our issue! (It is pretty much impossible to get find older versions of Chrome).

Since we had our site running on a developer’s machine and reproducing the issue, all we could do was remove piece by piece of Html/CSS/JavaScript until we could discover what was causing the issue.

After some time, it was a lengthy process as each test would result in a reboot, when we removed a resource reference to a font our site used, which was actually a Google Font on their CDN, it suddenly stopped BSODing. We added it back and voila; it crashed.

A reference to a Google Font on our site was giving our customers Blue Screens of Death.

After some research we discovered it was a Chromium bug which affected all versions of Windows (only), as Chromium/Chrome were working on native font rendering for Windows. Google were very quick to patch this issue, however, if someone was stuck on an older version then it would still be an issue: there wasn’t anything we could do about it but inform our customers to ensure they are on the latest version of Chrome.

I learned a few lessons during that day:

  • bugs can happen anywhere and cause damage that you can’t imagine;
  • bugs aren’t always in your control: we didn’t write bad software to crash our customer’s machines – this wasn’t tied to a particular release that we did. You can’t just test changes to your site and expect it to be okay;
  • you can’t find every bug: to find this bug we would have had to constantly check our upcoming and production site against every upcoming version of Chrome on every operating system. Chrome isn’t like IE with releases every few years, you’d almost need a full time tester just to perform this role; and
  • a website can blue screen of death your laptop.

Author: Alister Scott

Alister is an Excellence Wrangler for Automattic.

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