[vkou]

No, the risk is that Robinhood's users won't be able to pay, which would put Robinhood on the hook. The settlement house will only trust Robinhood as far as Robinhood has collateral.

This is not a problem for most stocks, because they have a balanced flow of orders (for every one of RH's customers buying, there's another one selling, which raises and then lowers their collateral requirement.) This is not a problem for bigger brokerages, who have larger cash reserves, to keep as collateral.

The tl;dr is that if you're going to do an uncoordinated mob short squeeze, with unprecedented volume and volatility, don't use a discount broker to do it.