There is an important thing to do before leaveing a coins-country ,which use many coins in daily life,like Singapore Thailand and Malaysia is to used all those metal before go to other country. After that ,It's becomes useless except some limit usage in a border town area.
One Exception in The SouthEast Asian countries (or the world)is Brunei,a small sultan-state but oil riches,which use some Singaporean coins in normal circulation because Brunei pegged their own dollar to Singaporean Dollar(SGD). Always! Then some people start to uses Singaporean coins in daily life .If you have a chance to visit Brunei then dont be surprise that a vendor might give your changes back in some SGD coins!
Several years back.I was standing puzzles at a foreign money exchange booth in KL and look around for the Brunei's rate but not found any until someone kindly enough to tell me about the arrangement between two countries.