Wikimedia Bugzilla is closed!

(In reply to comment #0)
> After watching Roan deal with this on IRC today (see r90960), there should be a
> way (in debug mode, or maybe when running E_STRICT) that a warning is displayed
> when addModuleStyles overwrites a previous style.
Impossible, marking WONTFIX.

The cause was something like:

$someglobal = array(
'foo' => stuff1,
'bar' => stuff2,
'foo' => stuff3,
);

Of course $someglobal['foo'] will end up being set to stuff3, and nothing will be able to detect that it was very temporarily set to stuff1 first.