Bug fix: Undefined index: title in drupal_check_module() line 986 of /foobar/includes/install.inc

This will set $requirement['title'] to the empty string whether it was already set or not, and I doubt $requirement['title'] will always be unset. How do you reproduce this error?

This would mean:

             $message .= ' (' . t('Currently using !item !version', array('!item' => $requirement['title'], '!version' => $requirement['value'])) . ')';

Has never $requirement['title'] which is senseless

I would use isset, because the title "0" is a valid title, but its a boolean false.

I don't see how this can possibly be a critical bug, or how it's necessarily even a bug in this function at all? As documented in http://api.drupal.org/api/function/hook_requirements/7, 'title' is always supposed to be returned, so this is occurring due to a bug in someone's implementation of this hook, not the code here.

Now what I also don't understand is the existing behavior of the function :) Why is there a drupal_set_message() in an API function like this at all (seems like that should generally be up to the caller), and why in the world does it try to assume that all error messages returned by hook_requirements() make any sense when printed out in the format "Currently using !item !version..."???

why in the world does it try to assume that all error messages returned by hook_requirements() make any sense when printed out in the format "Currently using !item !version

Hm, looking a little closer, I guess it's pseudo-documented in the hook that at install time, it's only supposed to return things like library version numbers and such... but it's still pretty weird.

I guess that's not for us to change here though. In this issue, let's just figure out where the hook implementation is that doesn't define a 'title', and fix the bug by defining one there.

Looks like everything in core defines a title. The issue creator should report which modules they have installed.