I once changed a piece of code that was referenced by every page on the customer facing site (10's millions of visits a day) to use a new function that someone

I once changed a piece of code that was referenced by every page on the customer facing site (10's millions of visits a day) to use a new function that someone had previously written (and was called on 1 page in the site). I mistakenly didn't really look to closely at the implementation of the function, and didn't realize how badly it's caching strategy was designed. When this code was deployed it instantly caused a thundering herd on our cache servers bringing the site down for about ~40 seconds. I felt TERRIBLE about it.