Twitter is experiencing some issues, users are reporting seeing "Twitter is over capacity" from time to time. The site sometimes works, but the issue is lingering. It's been happening for a while, but Twitter is only now acknowledging it.
"Some users may be experiencing issues accessing Twitter. Our engineers are currently working to resolve the issue," Twitter said on its status blog, but provided no useful information.
It doesn't look like a major problem and it should be fixed soon enough; but if you're encountering this, you should know that you are not alone.
Twitter doesn't have a problem with staying up anymore, the Fail Whale was a daily occurrence for a while a few years ago. Now, it's more an occasion for users to remember how bad it used to be than an actual problem. We'll update when there's more info.
UPDATE: Twitter fixed the issue, though it didn't provide too many details on what happened.
"We experienced intermittent issues affecting web and mobile users, globally, between approximately 7:00am and 9:50am PST," it explained. "This incident has now been resolved. We apologize to users who were affected by this, and we’re working to ensure that similar issues do not occur," Twitter said.