Several hooks (like e.g. PageContentSaveComplete) are currently called with parameters passed by reference, even though this is not necessary, not required per the specification of the hook, and actually dangerous: in many cases, such parameters cannot safely be replaced by a hook handler.
This behavior is probably a relict from PHP 3 times, when objects would be copied if not passed by references.
NOTE: this cannot be changed without first changing the signature of all hook handlers. Failing to do so will result in warnings like
Warning: Parameter 1 to AbuseFilterHooks::onPageContentSaveComplete() expected to be a reference, value given.