Let's say we want to test that after we successfuly send POST request, we can GET the same data on another endpoint. Pretty common for a CRUD-like apps.
Let's further assume our entity after creation from the POST request payload will result with a primary entity (e.g. Book) and child entities (2 Authors). We're using the constructor to create the entity (no setters) and connect everything:
$tags = .. // result is Tag[]
$book = new Book($title, $something, $tags);
...
$em->persist($book);
$em->flush();
If there is an issue with the Entity relationships configuration (e.g. missing cascade: persist) the EM flush call will
be successful (no exceptions) but only the primary entity will be actually flushed to the database, and the children
will not as they are not managed entities.
If we're using simulated requests, they are based on a stateful instance of your app, meaning the same Kernel and the same
EM object instance will be used for both operation. Since the EM caches the objects, calling find after flushing the
entitiy will return the children attached, since it will not perform a DB lookup.
In the reality, first request will create only the parent, and calling find on a fresh EM instance will lookup the DB to
hydrate the entity, and because of the bug with the entity configuration, children will not be there.
API test based on real HTTP requests will immediately discover this, while tests running with simulated requests will not.
There is a bug with entities configuration that will go under the radar with simulated requests and will cause tests with real HTTP requests to fail immediately due to the unintended behavior.