I've wasted a great deal of time today trying to figure out what was
causing issues in resource loader, whereby execution of JS seemed to
randomly abort without any errors. It turns out that a harmless call to
jQuery's .data() within RL was the cause. But not because of mediawiki
code... it was because of the FireQuery Firebug extension, which somehow
overrides or modifies jQuery.data in the execution environment. And not in
a good way, since it just seems to silently abort execution of the JS that
was calling it.
I'm not sure if having Firebug open was necessary for issues to occur. At
any rate, considering that FireQuery hasn't been updated in over a year and
that the author stated that he can't maintain it anymore , it seems like
sane advice to disable it.