Police officers are human and make mistakes. This is something we should expect and allow for. If a police officer makes an honest mistake, they should admit it so that everyone can learn from that mistake. This way, we can have more trust in our police.
We don't expect our police to be perfect! However, what we should never tolerate is police officers lying or conspiring to manipulate evidence in order to "beef up" their case or protect themselves from perfectly reasonable criticism.