Here at Grooveshark, we ran into a bug once where users in some countries couldn't perform searches. It took a while to figure out, but eventually we discovered

Here at Grooveshark, we ran into a bug once where users in some countries couldn't perform searches. It took a while to figure out, but eventually we discovered that it was a problem with the name of the API call the site makes to get search results. Originally the call was named 'getSearchResults'. Over time, we made some non-backwards-compatible changes to the search call, and following our usual naming standards, named the new call 'getSearchResultsEx'. Additionally, for ease of debugging, when we hit the API endpoint, we add the method name as a GET param. Makes it much easier when scrolling through Firebug's network tab to find the API request you need to examine, since they all hit the same php script. It turns out that in some countries, you can't visit a url that contains the word 'sex'...and the filter is case insensitive.