If you have a Pixel 3 or Pixel 3 XL and you run your battery all the way down to 0, when you plug it in you might get an endless bootloop.
I have found 2 ways to fix this. The first is to press the down button to enter the bootloader. This will allow the phone to charge. Allow it to get to 5% and it will boot normally.
The second fix is to get a charger with a higher voltage rating. The bootloop seems to only happen if the phone cannot fast charge.