I was looking at the Twitter StreamGraph for 'Star Trek' a little while ago and noticed an unusual pattern. There was a peak caused by many users sending the exact same tweet which contained a long list of trending hashtags that were otherwise unrelated - #googlefail, #whyitweet, #hubble, #star trek, #gmail etc. The tweet actually does vary slightly in that a different ow.ly URL is used but they all lead to the same place. It's obvious twitter spam carefully constructed to catch the attention of people following the trending terms.
Here are snapshots of two of the accounts showing that their last 6 tweets were identical. They have different numbers of followers with the one account acquiring an impressive 924 - more than I have. Presumably there is a large set of spam accounts and many follow each other. Other characteristics that seem to suggest spam besides the redundancy are no evidence of @replies and the fact that every single tweet seems to mention a product name and include a link.
I suspect it wouldn't be too hard to detect these algorithmically.