Is that the case here or is there some step I missed that would have prevented this from happening? I ask because I don't want to repeat a mistake in the future.

I recognize that locking the bootloader may have been the last straw for some present flaw - like a weak engine finally giving out after long service. When it finished and tried to reboot, it would only go the fastboot mode with the message 'ERROR: Slot Unbootable: Load Error'. While the device was in fastboot mode I issued the command 'fastboot flashing lock' and followed the phones on-screen directions. When it finished and tried to reboot, it would only go the fastboot mode with the message 'ERROR: Slot Unbootable: Load Error'. Today I decided to re-lock my bootloader. While the device was in fastboot mode I issued the command 'fastboot flashing lock' and followed the phone's on-screen directions. Today I decided to re-lock my bootloader. No custom ROM or kernel - stock everything, though I did have Magisk installed. Now when I try to boot, I don't get the verified boot screen, just the bootloader with 'ERROR: Slot Unbootable: Load Error' The bootloader no longer enumerates as a USB device, so I can't try reflashing the bootloader or flashing it to slot B (I am on slot A) or anything else. The phone had an unlocked boot loader with the latest OS (Pie) & security updates (November). What I want to know is if I did this or if this was something I couldn't have foreseen. Essentially I'm in the same situation as this poor soul (and many others, from the results of searching for this error in the forums).

Okay, so my phone is bricked and there's no recovering it.

Google pixel slot unbootable load error