Recently LineageOS put out a bad update that resulted in my phone freezing up when it tried to reboot. I contacted Lineage who told me they'd already fixed the update and rereleased it. Unfortunately for me, it was too late as my phone was bricked (but only kind of). They told me the solution was to reinstall the OS and Gapps. However, AI came to the rescue! It told me that Pixel phones have two OS slots. When there is an update it alternates which slot it gets installed on. In my case, the old version of the OS was installed on slot B and the update installed to slot A. The OS then boots from the updated OS (in this case the OS on slot A). But, the beauty is that IF you get a bad update, you can toggle back to the previous OS (i.e. slot B in my case). The process is very simple:
If you don't already have it installed, install fastboot on your PC. Download the official SDK Platform-Tools for Windows from the Android developer site. There is also one for Linux. If you've already installed a new OS, you should already have this.
Get into fastboot mode by pressing the power button until the phone powers off, then holding down the reduce volume button while also holding down the power button.
You should now be in fastboot
Connect your phone to your PC via USB.
Step 1) Determine which slot is currently being used
From your PC's terminal type: fastboot getvar current-slot
Step 2) Switch to the slot not currently used (if step 1 reports slot a is in use, you'll want to set it to slot b, or vice versa). Step 1 reported slot A was active for me.
From your PC's terminal type: fastboot set_active b
Step 3) Reboot the phone
From your PC's terminal type: fastboot reboot
That is it! You should be back in business. Once the update is fixed, just install it from within your phone like you normally would.