Twitter announced the cause of its mass, site-wide outage on Thursday afternoon on its TwitterComms account, writing: 

"Today's outage is due to a cascaded bug in one of our infrastructure components. We'll provide updated information soon."

A Twitter spokesperson further elaborated to TPM that "a cascaded bug is a bug that’s not confined to a particular software element, but one that 'cascades' into other parts of the system."

No doubt #cascadedbug will be trending shortly.