Twitter’s downtime June 21, at around 9AM PDT, was due to a bug that affected multiple parts of the service and consequently had a worldwide impact. The company has since apologized, adding the service is almost consistently online.

“This wasn’t due to a hack of our new office or Euro 2012 or GIF avatars … A “cascading bug” is a bug with an effect that isn’t confined to a particular software element, but rather its effect “cascades” into other elements as well,” a blog post from the company’s vice president of engineering - Mazen Rawashdeh - said. Exactly what that bug was hasn’t been revealed, but it was enough for Twitter to revert to a more stable version of the micro-messaging service.

The recovery process began at around 10:10AM PDT, before what Twitter described as “full recovery” beginning at 11:08AM PDT. The service dropped for the second time around 10:40AM PDT.

Twitter has been pretty reliable in 2012: its last major crash was on New Year’s Day, which was due to a traffic overload. When Twitter began, crashing was a common occurrence but investment into the service has increased reliability. A “fail whale” appears when the service goes down, which has become somewhat mainstream due to Twitter’s flaky history. It’s even a worldwide trending topic when the service is interrupted, which almost seems ironic.

In fact, Twitter has been so reliable that the service has been up for around 99.96-99.99 percent of the time. Basically consistent, aside from the last 20 seconds of a day according to Twitter. The impression from the Twitter blog seems to be that these kinds of outages are become one-off events.

The service hasn’t buckled under the pressure of Euro 2012, which is ongoing, or WWDC. Apple events are worldwide trending topics, as are events such as the Electronic Entertainment Expo.

Twitter has advised users to monitor the site’s status blog for information on when the service would come back online, around two hours after the downtime began. The blog post came after a tweet citing the “cascading bug.”