My company has a project that is funded by NIH, and as part of our project work we are collecting tweet data from the 1% API stream for use in epidemiological research. Up until last week, the python (tweetsream-based) application that was collecting the data was running on an AWS EC2 compute instance. Without any warning or comment from Twitter, we started getting the '401 Unauthorized' error, and our data collection requests were blacklisted.
Twitter's support system seems designed to prevent users from submitting a support ticket. After 4 or 5 tries, I finally managed to get a ticket into their system, but only received robo-responses that did not address the blacklisting issue. Finally, a couple of days latter my colleagues and I were able to determine (empirically) that some or all twitter apps being hosted by AWS were being similarly blacklisted.
My solution was to move the data collection application to a non-AWS server.
I realize that the 1% API sample data is free, but I don't really think that justifies Twitter presenting an impenetrable support system to its user base.