Three cases of implementation aspects have been mentioned that may change the validity of chains under certain circumstances: buried soft forks, not verifying BIP30 after BIP34 is active and the existence or removal of checkpoints. However, none of these will influence the accepted main chain. Pieter Wuille believes that checkpoints should be eliminated because they are misunderstood as a security feature rather than an optimization. Buried softforks do not have this problem.