Google has a number of neat case options in the Google Store for this year's Nexus phones. If you've been planning on picking up one of the official cases like the "Adopted" model, you might want to pay attention to the buttons. It actually makes taking a screenshot almost impossible. If you've already got one of the official cases and have thus far been under the impression you are losing your mind, you'll be happy to know it's not you.
Taking a screenshot on the 6P is the same as any other (non-Samsung) Android device—just hold power and volume down. The volume and power buttons on the Nexus 6P are on the same side and fairly close to each other. The buttons on the Adopted case are just little bumps in the rubber that hit the physical buttons when you press them. The problem is that the case itself pushes inward when you hit the power button and volume down to take a screenshot. This will trigger the volume up button in addition to volume down, which blocks the screenshot and instead captures a bug report.
This strange design foible has been pretty much universal for us. I've seen it on my unit with the "Nexus 6P Case" (very similar design), Artem has it with the Adopted case, and several friends of AP are seeing the same thing. These aren't bad cases, but this one flaw is incredibly frustrating if you take a lot of screenshots. I suspect it's also going to pop up with any cases using similar button designs, so use caution when buying a case for the Nexus 6P. The Speck case Google is selling has a less flexible plastic shell and separated buttons, so I bet it works fine.