transaction problem

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Hi list!
I've a problem with transaction
I initiate a transaction with "start transaction". Soon i execute 
following query:
* update t1 set t=12; and by I complete I execute rollback but when I do 
it I obtain the following thing:
"non-transactional Some changed tables couldn't be rolled back" and 
update I execute myself updating the data
Why do it?
I'm use MySQL 4.1.20 and MyISAM tables

Thanks,

Pablo