The following test case from update.test updates fewer rows than it should:

create table t1 (f1 int);
create table t2 (f2 int);
insert into t1 values(1),(2);
insert into t2 values(1),(1);
update t1,t2 set f1=3,f2=3 where f1=f2 and f1=1;
select * from t2;
drop table t1,t2;

Testing the result of the update via

select * from t2;

returns:

while the correct state after update is:

In addition the number of updated rows is reported as 2 instead of
the correct number 3 (the sum of updated rows for both tables).