It was a rough day for Twitter, which had not one but two service outages within a few hours of each other. The service first went down between 8:30 a.m. and 10 a.m. Pacific time on Thursday morning. About an hour later, Twitter went down again, leaving its users with slow service for much of the day. The company blamed the problems on a “cascading bug” in its system. UgNazi, a hacker organization, claimed responsibility for the disruption. Twitter, which has had reliability problems in the past, has been trying to work on its consistency issues.