Fixed that this sequence didn't produce an error as it should have:

CREATE TABLE t1 (s1 CHAR(5) COLLATE latin1_german1_ci,
                 s2 CHAR(5) COLLATE latin1_swedish_ci);
SELECT * FROM t1 WHERE s1 = s2;

mysql-test/r/ctype_collate.result

@@ -530,3 +530,9 @@ latin1	latin1_swedish_ci	3	1
 SET CHARACTER SET 'DEFAULT';
 ERROR 42000: Unknown character set: 'DEFAULT'
 DROP TABLE t1;
+CREATE TABLE t1 
+(s1 CHAR(5) COLLATE latin1_german1_ci,
+s2 CHAR(5) COLLATE latin1_swedish_ci);
+SELECT * FROM t1 WHERE s1 = s2;
+ERROR HY000: Illegal mix of collations (latin1_german1_ci,IMPLICIT) and (latin1_swedish_ci,IMPLICIT) for operation '='
+DROP TABLE t1;

mysql-test/t/ctype_collate.test

@@ -148,3 +148,11 @@ SELECT charset('a'),collation('a'),coercibility('a'),'a'='A';
 SET CHARACTER SET 'DEFAULT';
 
 DROP TABLE t1;
+
+CREATE TABLE t1 
+(s1 CHAR(5) COLLATE latin1_german1_ci,
+ s2 CHAR(5) COLLATE latin1_swedish_ci);
+--error 1265
+SELECT * FROM t1 WHERE s1 = s2;
+DROP TABLE t1;
+		 
\ No newline at end of file

sql/item_cmpfunc.cc

@@ -249,7 +249,8 @@ int Arg_comparator::set_compare_func(Item_bool_func2 *item, Item_result type)
       We must set cmp_charset here as we may be called from for an automatic
       generated item, like in natural join
     */
-    if (cmp_collation.set((*a)->collation, (*b)->collation))
+    if (cmp_collation.set((*a)->collation, (*b)->collation) || 
+	cmp_collation.derivation == DERIVATION_NONE)
     {
       my_coll_agg_error((*a)->collation, (*b)->collation, owner->func_name());
       return 1;