One of the most infamous examples is the old AF-D Nikkor 50mm f/1.8 (as well as the AF-D 85mm f/1.8). Especially closed-down couple of stops, its bokeh is highly intrusive and distracting. Often to the point that it renders a photo unusable. Of course, the best solution is to use a lens that offers you crazy-good bokeh at all situations. It doesn't even have to be expensive (the Samyang 85mm f/1.4 is what comes to mind). But what if you have a not-so-great bokeh on an existing image?