1. [Doctrine\Common\Persistence\Mapping\MappingException] Class 'Your\SyliusPlugin\Entity\YourEntity' does not exist
When your plugin is AbstractResourceBundle and all your entities should be and actually placed at Model directory,
you have that exception (notice ...\Entity\... at exception rather than ..\Model\...)
for your Your\SyliusPlugin\Model\YourEntity.php that is weird.
This happens when you accidentally have your mapping file at src/Resources/config/doctrine/YourEntity.orm.xml (without ../model/.. part) rather than src/Resources/config/doctrine/model/YourEntity.orm.xml.
This issue may happen when you convert some of your application code (which use Entity directory) to plugin (which use Model directory).