Twitter users are reporting problems receiving tweets in their official Twitter apps this afternoon.
In a post to its official status blog, Twitter acknowledged the issue, saying: "Some users may be experiencing a delay with new tweets in their timelines. Our engineers are currently working on this issue."
A search for "Twitter app" reveals a lot of unhappy people, though people using clients like seem to be having no problems.
In recent days, there's been much talk of the end of the so-called "fail whale" in light of Twitter's performance having been quite reliable recently. Perhaps that talk was a bit premature.