i think it’s just bad UX. my understanding is that Citadel acts as the market maker for Robinhood. if it ever can’t provide liquidity for a trade, you need to handle that case explicitly. make it clear to the user where liquidity comes from and how much of it there is, otherwise they can’t reach for any cause besides “Robinhood (Citadel) decided to pull a switch”. order books used to serve that role: i’m not sure what the solution is in this era of HFT.