Piazza is a learning management system that allows students to post questions which are then answered by fellow students or the lecturer. One of the features of Piazza is anonymous posting where students can choose not to reveal their names. Unfortunately, correlations can be made through observing the style of writing, causing information leakage. Just to clarify (pun intended), it is no fault of Piazza though.
Update (27/08/15): Turns out that Queensland Police used the same exact technique to de-anonymise an administrator of a child porn tor site.