Hi, I live in a flat the uses hot water pipe linked to every flats in the block. I think its due to having old style heating system and hot water tank. Its all was removed and combi boiler was installed long time ago, no issues. Since then council started installing thermostat type bath tub water mixer. These cheap thermostat does not work properly. If you turn your cold water tap and neighbours thermostat in the bathroom failed, then hot water will run from neighbours boiler to your cold system. I realised this and changed thermostat to an expensive one, what fixed the issue.