When they submit an answer, they see "Answer Preview", and interpret this as being a preview of the correct answer. This inevitably leads to many emails to instructors of the form "WeBWorK shows my answer is identical to the correct answer, so why did it mark it wrong?"
Does anyone else experience this? If so, do you have any solutions? One thing I've done is I've changed the text from "Answer Preview" to "Your Answer Previews As" (inspired by similar wording used in gateway quizzes), which will hopefully make it clear to students that they are seeing their own answers.