Given the following service registered:

services:
    Acme\Foo: ~

Running the following will fail:

bin/console debug:container Acme\Foo

The reason being the correct way would be to escape the backslash:

bin/console debug:container Acme\\Foo
# or
bin/console debug:container "Acme\Foo"

I however find a bit tedious and unfriendly: I register a service by a class but if I copy/paste the class I won't find the service... Plus it's a bit annoying manually escape if there is a lot of backslashes. Maybe the debug:container command could be made a bit smarter to understand the AcmeFoo is similar to Acme\Foo.

Edit: The DX issue here IMO is that the simple copy/paste does not work (you need to escape the service name) and that can lead to confusion. Maybe the error message could help a bit with it