WebLocalizedString() in WebLocalizableStrings.m is not thread-safe because it performs no internal locking when setting or using the value of a static variable. Also, it calls -[NSBundle localizedStringForKey:], which is not guaranteed to be thread-safe.
To catch thread-safety issues, it would be useful to assert that this function is being called on the main thread.
Created attachment 48166 [details]
Patch to add an assertion.
Fixed in <http://trac.webkit.org/changeset/54367>