Romans 2:21

‘You therefore who teach another, do you not teach yourself?'

The question is sarcastic. They claim to teach others how to live, but they do not themselves live as they teach. Thus they seemingly fail to teach themselves.

‘You who preach that a man should not steal, do you steal?'

For example they taught that it was wrong to steal, something that was central to the covenant. And yet they themselves stole in all kinds of ways, by sharp business practises, and as a result of their contempt for the Gentiles, not considering theft from Gentiles as really theft. Paul no doubt had examples in mind.