Hi celt and RF Learner,
Actually BSC gives the handover command on the basis of two things 1.BSIC 2.BCCH,now suppose you have two sites(WITH SAME BCCH and BSIC)near one site,and if one site is defined as neighbour of that site and another one is not,THEN BSC will give handover command to the BTS to give handover,now as both sites has same same BCCH AND bsic So BSC will unable to find out which is the real neighbour and thats why handover failures will come.YOU can verify this in layer-3 messages under the name HANDOVER COMMAND,you will surely get the answer.
BR//
Yogesh_rf