If tl-setup fails to install a package, e.g. python-ldap (bug 7330), in text mode we print a warning about this and the setup continues when pressing enter.
In graphical mode we say:
> "Failed to resolve optional packages and dependencies.
> The configuration process will continue but some parts of ThinLinc
> might not work correctly. [...]"
But you can't press the forward button. You have to go back and un-check the checkbox and then continue. This is a bit confusing when we say that the configuration process will continue.
*** This bug has been marked as a duplicate of bug 4631 ***