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If you are shooting raw, this thought has probably never crossed your mind. However, I've noticed some folks who shoot JPG think differently, and when they start using Lightroom they can make a serious mistake.
Although it is never a good idea to edit your originals, some folks do for one reason or another. If you are used to shooting JPG and then editing your photos in a pixel editor such as Photoshop, you might open a photo, make a tweak, and click Save and then click Close so that your original now contains your new edits.
If you have that same mindset in Lightroom, you can run into trouble. Lightroom is designed to never alter the pixels in your originals. All the adjustments you make in Lightroom are simply saved as a set of instructions that it will only apply to copies of the original during some form of export (print, slideshow, Web, new file). The problem I've seen more than a few people run into is that they want to apply the Lightroom changes to the pixels in their original photo, so they decide to export a copy back to the source folder and overwrite the original photo. Don't do it!
Unfortunately Lightroom will let you try, but the results are tragic. During export, Lightroom needs your source photo to create the copy. When you point that export at your source folder, Lightroom will ask you if you want to "Overwrite" or "Use Unique Names." If you click "Overwrite," Lightroom will remove the source photo to make room for the new file, but it needed that source photo to create the copy! The result is that Lightroom removed the source photo but can go no further and lets you know an error occurred while exporting. At that point you will realize (with great sadness) that your source photo no longer exists. So please, please don't do that.
Thankfully Lightroom 2 does not allow you to overwrite your source photos. You can try, but it will not give you the overwrite option anymore. Here's hoping a future update to Lightroom 1 will prevent that option as well.