Completely unrelated but interesting case:
https://www.propublica.org/article/lord-of-the-roths-how-tech-mogul-peter-thiel-turned-a-retirement-account-for-the-middle-class-into-a-5-billion-dollar-tax-free-piggy-bank
Summary: Guy who founds Paypal (before it went public) is given stock with an ultra low cost basis as part of his salary contract. The cost basis is deemed $1,700 so he puts the shares into his Roth IRA. The company goes public shortly after that and those shares are worth well over $100m. He does this multiple times with different companies and his Roth IRA is now worth $5 Billion which he'll never pay $1 in tax on if he holds off on withdrawing.
Yes it's all strictly prohibited but the IRS never audited him. You would think a $1,700 Roth value going up 9 figures in one year might draw an audit flag. First, he was a principal in the company so that's a violation. Second, he contributed stock and not cash which is prohibited. Third he valued the shares at far below market value and the IPO disclosures of the firm outline they were below market value.