Upon reaching the configured memory_limit, I get a fatal error from the Debug component as it tries to handle the error. For example:

Fatal error:  Allowed memory size of 67108864 bytes exhausted (tried to allocate 28672 bytes) in /var/www/vendor/symfony/symfony/src/Symfony/Component/Debug/ExceptionHandler.php on line 239

I can fix this by increasing the amount of reserved memory in Symfony\Component\Debug\ErrorHandler:

self::$reservedMemory = str_repeat('x', 20480);

This allows the error handling process to complete and the OutOfMemoryException is thrown as expected, but there is currently no way to configure this value.