My professor and chair of the department traced back a supposedly anonymus course evaluation to me. How to proceed? At my large American university, we have the opportunity to fill out course evaluations. They are not mandatory and are allegedly anonymous. I wrote a negative review for Dr. Smith's course. I stand behind what I wrote and it is all true, but it was scathing and lengthy (over 1500 words) since I listed in detail almost every thing I didn't like and why, with specific examples. If it matters, my main complaints were that I consider his class to be too easy, his standards were too low, and that he took a long time to grade our assignments. 

I received an email from the chair of the department asking me to meet with him to discuss the evaluation. The content of my evaluation is all true, but I am embarrassed that my identity was revealed (or correctly assumed) since I would have been less harsh if I was aware the anonymity wasn't guaranteed. It wouldn't surprise me if they guessed I wrote the review, since I made by far the highest grade in the class and had actually complained about Dr. Smith in the middle of the semester to the chair since Dr. Smith allowed something unfair to happen during the midterm. (I was advised by other faculty to bring that specific complaint, so I hope it doesn't come off that I like to complain.)

What should I do and can anything negative happen to me over this?
Most probably nothing negative will come out of it. 

And there is also the chance that they want your opinion, independent of the evaluation (i.e., they did not necessarily guess that it is YOU who wrote these comments but given the fact that you talked to the chair about these issues, then the chair wants to discuss these with you.)

True, it shouldn't have happened the way it happened (to make you think that the evaluations are eponymous or that they have guessed the writer behind these comments and want to talk to you about that)  but most probably the chair and professor would like to discuss the issues for potential improvement. Just politely explain all these issues and make some suggestions on how they could be resolved and it should be fine.