1. What if after the first month or so on a project I realize that I don't really like research in general, or the project? This happens for many reasons, and how you handle this is very important. Remember that one of the things you would like as a result of the experience is a letter of recommendation from your mentor for your first job, graduate school, or professional school. You can get this even if you find out that research or the specific project is not for you.
You must handle the situation like a professional. Do not make it obvious that you are not satisfied with the project. Remember that the work is important to the mentor and that they agreed to provide you with research training. Keep your opinions to yourself. Show up as scheduled and work as hard as you can - you want the letter to say you are diligent and a hard worker. You can demonstrate this even if you know you will never work on a similar project again! Honor the time commitment you made in your original meeting with the mentor. If you promise a semester, an academic year, or a summer semester, do it, and do it well. They have planned their work with the expectation of having your assistance for that time period.
Several months before the end of that time period, you may let them know how much you appreciated the opportunity to work with them and the training you received, but that your research interests have changed and at the end of the agreed upon time period you will be moving to another lab or will instead be concentrating on your studies.
2. What if my classes are more time consuming/harder than I expected and I am having a hard time keeping my grades up and doing the research? Research should not negatively impact your grades. If you find yourself in this position, discuss it with your mentor. The two of you will come to an agreement about how best to handle it (reduced hours? cessation of project? start the following semester?). The key is to go to them as soon as it appears to be a problem. Don't wait until you are in trouble in class or not showing up for research.
3. What if I have a personal/family/health problem and can't do the research as I had hoped? Sometimes life happens, and you find that you are unable to meet your committments. Once again, the key is to discuss the situation with your mentor immediately. The worst thing you can do is just disappear. Talk to them, explain why you cannot do the project, and you will find a workable solution together.