Nobody makes a mixtape to make a completely cohesive whole. It’s patchwork by nature, which means that even if your overall aim is to have a set of music that fits together, at some point the seams will show. So you might as well let them show deliberately here or there. Follow a series of ballads with the raunchiest rap song you know, let a groove of danceable tracks collapse into a jagged post-punk song. In radio, disc jockeys are warned away from letting “trainwrecks” happen on the air that might annoy and put off audiences. But for an audience of one, whether it’s you or someone else, you can afford to take some chances and let some sounds clash for dramatic effect.