Twitter suffered a double outage that rocked Twitter last night as users around the world reported significant down-time and slow service across the website and mobile applications.

Twitter blamed the the disruption on a “cascading bug” in one of its infrastructure components and it became  ”inaccessible for all web users, and mobile clients were not showing new Tweets”.

Twitter said the problem was wasn’t due to a hack or its new office or Euro 2012 or GIF avatars as some had speculated.

Mazen Rawashdeh, VP, Engineering (@mazenra), at Twitter, said: “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. One of the characteristics of such a bug is that it can have a significant impact on all users, worldwide, which was the case today. As soon as we discovered it, we took corrective actions, which included rolling back to a previous stable version of Twitter.

“We began recovery at around 10:10am PDT, dropped again around 10:40am PDT, and then began full recovery at 11:08am PDT. We are currently conducting a comprehensive review to ensure that we can avoid this chain of events in the future.”

The outage took many back to the earlier days of Twitter when down time and the Fail Whale were common problems. However, Twitter said that for the past six months it has enjoyed its highest marks for site reliability and stability with at least 99.96% and often 99.99%.

“In simpler terms, this means that in an average 24-hour period, twitter.com has been stable and available to everyone for roughly 23 hours, 59 minutes and 40-ish seconds. Not today though.  We know how critical Twitter has become for you — for many of us. Every day, we bring people closer to their heroes, causes, political movements, and much more.”

“It’s imperative that we remain available around the world, and today we stumbled. For that we offer our most sincere apologies and hope you’ll be able to breathe easier now,” Rawashdeh added.

How was the outage seen by users? It varied, but one user, Arghya Roychowdhury, put it this way:

However, although Twitter ruled out a hack one group hacking group, UgNazi, claimed credit for the service disruption in an email to Reuters.

It said it  had launched a distributed denial-of-service (DDoS) attack against Twitter “because of the company’s support for the Cyber Intelligence Sharing and Protection Act”.

Its claim and credit for the attack were dismissed by security experts as “mere coincidence”.