When you add a Custom Block with select attribute and NotBlank validator, that contains the choice with value 0 and this choice selected, validation will fail.
Steps to reproduce
1. Clean installation of eZ Platform 2.5 / 3.0
2. Create a new custom block testblock in ezplatform.yml:
3. Create a template file for the block
src/AppBundle/Resources/views/blocks/testblock.html.twig (3.0: templates/blocks/testblock.html.twig)
4. Create a new Landing Page, put Test Block inside, and choose "Option 1".
Block validation will fail.
Block will validate successfully.