Next up: nuts. Almonds, to be exact. In one of our most demanding blender tests, we see if our test subjects are capable of grinding almonds down into flour, and then on into almond butter. It's no easy task.
The Designer 725 is designed to tell you if something's wrong. In this case, we encountered some "cavitation," which simply means that the almond flour/butter was stuck to the sides, away from the blades, creating a cavity around the bottom. Basically, the blender is telling us we need to stop and stir.
That black rubber gasket surrounding the spindle was the problem. The Blendtec rattles around a bit when you use it, bringing the rubber into contact with some fairly ferocious friction. That caused it to burn up and shred. Yuck.
In fairness, for nut butter recipes, Blendtec recommends using its Twister Jar, with built-in side scrapers. We tested it out, and it does indeed work brilliantly. However, if you want one, you'll need to pay an extra $120.