I know of an elder who was and still is, very well known around where I live. He and his wife were both full time pioneers but had a small shop. When their business hit hard time, he liberated some of the contributions from the congregation regularly for several years to help themselves out. He was also in charge of the accounts at his KH and at Circuit and District Conventions.
He was found out when he started to get greedy. He was not reported to the police but he lost his priviliges for about two years. Last I heard he was back to being an elder and giving talks at assemblys. So, it does happen. Though he's the only case I've heard of.