twitter/twitter-text-conformance — pull request #58 by kungfumike (2013-09-13 20:41:49+00:00)
Title: Twitter actually shows a mention if it is preceded with any mixed

case of the characters "r" and "t".
https://twitter.com/sono_inf/statuses/310385896321056769 provides
a lower case example. Extracted streams from Gnip and Datasift
confirm this behavior as well.
This work done on the dime of: Simply Measured simplymeasured.com