Working at a fortune 500 way back in 2011, I was assigned a ticket which was about asynchronously rotating an ad block at the top of a public search page on an

Working at a fortune 500 way back in 2011, I was assigned a ticket which was about asynchronously rotating an ad block at the top of a public search page on an extremely busy site. There was a tiny bit of logic in there if I remember correctly, relating to when and whom to show the ad to. I implemented it, tested in dev, assigned it to QA, QA approved it and rolled it out that afternoon. The next morning when I showed up to work one of the other team leads walked straight up to me as I entered the campus, laughing hysterically. Turned out that in production it behaved differently. The ad appeared, correctly. Then doubled every couple of seconds,1 ad, 2 ads, 4 ads and so on, until the client browser exploded into flames. As luck would have it there was zero consequence, only chuckles all over the place. It was something to do with order of inclusion of a mad number of js libraries which differed in production and went unnoticed. The End.