There might be some changes in SW2012 which I am not aware of or there might be an option I accidentally turned on/off

I work on a part and make some changes on it. Then I save and close the part

Since the assembly file is big (about 1000 components) and I make no change on it so I close the assembly without save it ( It'd take a while if I save the assembly) - then if I close the assembly wihout saving it  and open the part,  then all the changes on the part is gone - I don't think this works same as previous releases

Am I correct or missing something here?

Thx