The effective fulfillment of the lender-of-last-resort function of the central bank requires that during a liquidity crisis the central bank lend freely to an institution that is illiquid but not  insolvent (if assets can be held to maturity), against collateral that would be good during normal times but that may have become illiquid during disorderly market conditions, and at a penalty rate.  The requirement that lender-of-last-resort facilities are only offered on punitive terms is key to the minimisation of moral hazard and thus to discouraging future imprudent, reckless lending and borrowing.

The same principle of providing assistance to illiquid but solvent institutions only on punitive terms applies when market illiquidity rather than funding illiquidity is the problem facing the troubled private financial institution and the central bank intervenes as market maker of last resort.

Any illiquid assets the central bank accepts as collateral in repos, at the discount window or at any of the more recently created special liquidity facilities, such as the Term Securities Lending Facility and the Primary Dealer Credit Facility in the USA or the Special Liquidity Scheme in the UK, must be valued or priced in a way that ensures that the transaction does not involve a subsidy from the central bank to the borrowing institution.  The repo, collateralised loan or swap should earn the central bank an appropriate risk-adjusted rate of return.  The same would apply if the central bank purchased illiquid private securities outright from a financially challenged private financial institution.  There is nothing wrong in principle with the central bank taking credit risk onto its balance sheet, as long as it earns a rate of return that adequately compensates it for that risk.

There is a growing suspicion in the markets that the ECB is subsidizing some euro area banks that are eligible counterparties at its discount window (the Marginal lending facility) or in repos, by overvaluing or overpricing illiquid collateral offered to the Eurosystem by these euro area banks.