Notice the comments in this answer for Combinatorial proof that binomial coefficients are given by alternating sums of squares? Two of the comments begin with the funny-looking string "š:". The author of the answer is Luboš Motl.
What I think happened is that the comments originally began "@Luboš:", before the new feature was introduced to automatically remove @user alerts to the author, but apparently that feature got confused when faced with the "š" in "Luboš".
I assume this is a bug.