Twitter warned users that a failed upgrade Monday night has left the site unusable for many.
The problem began past 8 p.m. PDT, the company said, and might last for hours. "Our infrastructure and operations engineers are currently working to resolve this. The site could be down until approximately 3 a.m. PDT," company spokeswoman Carolyn Penner said.
The Twitter status page blamed the problem on "the failed enhancement of a new approach to timeline caching."
Signs of problems included the infamous "failwhale" image indicating problems, another image signifying "Something is technically wrong," missing or late tweets, duplicate tweets, and a Web page that didn't work at times.
Twitter has generally fared better than its early days of popularity, when "failwhale" sightings were common. Although availability of the site and service have improved, though, the repercussions for outages today are higher: Twitter is more seriously used by companies for customer communications and marketing, more ordinary people use it in their own lives, and competitors such as Facebook and Google Buzz are more serious.