There's this famous line item which is called something like "securities sold but not yet purchased", e.g. with their market maker privilege Citadel can create infinite synthetic shares out of thin air to facilitate that a trade happens, but they have abused this privilege on a very large scale which has created an idiosyncratic risk to all stock market investors.

Due to regulatory capture of the SEC this risk has not materialized in an overall market crash, but they have done numerous accounting shenanigans and deadline extensions to give Citadel more room to breathe.