Technically you can't overcook most food because the temperature never rises above what you set; however, that doesn't mean that you can't leave it in the sous vide bath too long, you absolutely can. This is especially true of sous vide meats, like chicken, pork, and beef. The texture changes. So, while the internal temperature is exactly (or close to) what you set the sous vide appliance for, the meat fibers break down more after longer sous vide times. This can lead to mealy and soft meat textures that are unpleasant. There is much bigger margin of error though than stovetop or oven cooking, so as long as you are within an hour or so of the optimal sous vide timing, you'll be just fine.