1. Time is valuable. If your internship coordinator brings you on board, they are putting time into helping your learn. Don't waste people's time.
2. If there is a problem of ANY kind, there should be a sit-down conversation or phone conversation with your employer where you both try to come up with a solution. In most cases, a solution can be reached. If you just quit, no solution can be reached.
3. Don't burn bridges! You have to remember the EVERYONE KNOWS EVERYONE mentality.
4. You've lost a reference and a line on your resume. Once you leave an internship, even if you were the best intern for the few weeks you participated, you cannot email that person for a reference. Your reliability is now an issue. You also don't want to put this on your resume because you didn't complete the internship and you don't want a potential employers to ask too many questions about it.
5. You must learn commitment. We often commit to things before we have a chance to think them through. Employers always say that students have commitment problems and often "flake" out. Don't be the student that flakes out. Find a way for your internship to work within your schedule, work with your employers to make sure you HONOR YOUR COMMITMENTS. This is crucial at your internships and in life.