What are the biggest mistakes students make when writing comparative essays in examinations with a limited time (30 minutes)?
Most errors are due to students not knowing the material. There are no test-taking strategies that will help if you haven't done the assigned readings (the full assigned books -- not just study guides).
For students who do know the material, the major problems arise from not planning out answers. Often, students will write too much about one part of a question and not finish, or be so busy trying to write down everything they know even vaguely related to a given topic that they won't focus on the main point of a question.
The best solution is to read the question thoroughly, and then write an outline of the main points you need to cover. Make sure to cover each point only to the depth appropriate for the test -- don't try to write 1000 words for a short answer question. Show that you can judge what is important by responding directly to the main focus of the question and then move on or proofread.
In a comparative essay, make sure you have a comparative thesis, and cover both works equally, and have a comparative conclusion.
A short well-organized answer is preferable to a disorganized mass of random facts.