I stumbled upon this. In applications/forums/sources/Topic.php on line 1861: 

$comments[ $_hash ]	= $this->_comments( $class, $limit ?: static::getCommentsPerPage(), $offset, ( isset( $class::$databaseColumnMap[ $order ] ) ? ( $class::$databasePrefix . $class::$databaseColumnMap[ $order ] ) : $order ) . ' ' . $orderDirection, $member, $includeHiddenComments, $cutoff, $includeWarnings, $extraWhereClause, $includeDeleted, $includeWarnings );

You pass on $includeWarnings twice. 

You send 11 parameters, but the function _comments in system/Content/Item.php "only" takes 10. 

(As a sidenote: I spent far too much time debugging an issue I had, which ended up being my fault, due to the amount of parameters sent in these functions making me miss one and causing lots of headache)

Thanks, I've submitted a branch to resolve that issue.