Many moons ago I worked for a company that did some outsource work for another company who asked for responses to an anonymous survey.
So I responded.
1. I shouldn't have gotten the survey as I didn't work for the company who sent it, but nobody at either company was very smart or careful because it went to everyone.
2. I misread the email and didn't notice it wasn't my company asking for responses it was the company we did outsource work for.
I made some pretty tepid constructive criticism. So did a couple other people.
The next day we were in a conference room with people we never met before angry that we responded to the survey. They hadn't realized we were actually sent the email just like everyone at the company (like I said these were not smart people) ...
It also raised the question about how anonymous this survey was. Of course it wasn't, you could see in the URL when responding your name, email address, etc ;)
But also a bit shocked how many people feel that participating is a good idea.