As more people join Developer Community, and with increasing efforts to promote code sharing, I'd like to draw fresh attention to this post I wrote a year ago. It spotlights a feature within the class compiler which is both useful and dangerous. When importing code (e.g. from an XML export of classes received from someone), it's worth considering the risks.
Even if that post doesn't seem relevant to you at the moment you may wish to note it for the future. A handy way of doing this is to click the star icon at the end of it.