Misrepresenting a source
When you use someone else's ideas in your writing, you are expected to represent those ideas clearly and accurately. If you don't understand something you've read, don't try to incorporate it into your essay. If you try to incorporate ideas that you don't understand, you run the risk of taking those ideas out of context or making the case that an author says something other than what he or she has actually said. In addition, if you don't understand a source, you are more likely to blur the distinction between the source ideas and your own, which could lead you to unintentional plagiarism. The best way to prevent this is to make sure that you understand every idea that you are using in your paper, and to talk to your instructor about areas of confusion.
Ignoring sources found late in your research process
If you have already written your paper and you find a source that makes the same argument you've made in your paper, don't ignore it and submit the paper anyway, assuming no one will notice. Instead, consult your TF or professor before deciding what to do. It may be that the source's argument isn't quite the same as yours, or that you can revise your argument to differentiate your ideas from those in the source. If you discover the source at the last minute and you don't have time to ask your instructor, add a footnote explaining when you found the source and how it compares to your own paper.
Turning in the same paper for more than one class
Harvard has a very clear policy on using the same paper for more than one class (see Harvard Plagiarism Policy). Although a paper you write is clearly your own work, you are expected to produce new work for each course so that you can incorporate what you have learned in that course, and so that you can receive credit for doing work in that course.