Strange transformer fault.

Have been using some 300va 2x12v output tx,s to supply heated takeaway bags.The two outputs are marked 0 and 12v each,so am linking

0-0 and 12v-12v to give a single 12v output.Out of a batch of 20txs this works fine,but on 2,the 5A primary fuse blows as soon as the

secondarys are linked.Could this be caused by one of the 12v windings being in the opposite direction to the rest of the batch?

                                                                                                             Thanks for any advice,

                                                                                                                         Regards,

                                                                                                                               Hz