Fortunately, the Fed's interventions seemed to work. The repo rate returned to its usual level, close to the federal funds rate,
which in turn is within the range targeted by the Fed.
Even so, the turmoil raised questions about how it plans to handle future cash shortages.
The mere prospect of them marks an important shift for America's financial system.
Before the financial crisis the Fed controlled the federal funds rate using a "corridor", with a ceiling and a floor.
Banks with too little cash could borrow at the ceiling rate.
But there was no compensation for extra cash held at the Fed (the floor interest rate was zero).
To keep interest rates precisely on target the Fed used "open market operations",
swapping Treasuries and cash to control liquidity in the banking system.
Six years of quantitative easing changed all that. To push down long-term interest rates,
the Fed bought vast quantities of long-dated Treasury bonds. Its balance sheet ballooned to $4.5trn.
The holders— mainly banks—ended up with mountains of cash. To keep market interest rates at or above the policy rate,
the Fed was authorised by Congress to raise the floor from zero, compensating banks for their cash that it held.
The ceiling became redundant, as did open market operations. Only the floor mattered.
But banks' cash piles have dwindled of late.
Since late 2017 the Fed has been reducing its balance-sheet by not reinvesting all the proceeds when its assets mature.
The balance-sheet shrank from $4.5trn in 2017 to $3.8trn in June this year.
Moreover, a wider budget deficit means the Treasury has had to issue more bills and bonds.
So far this year it has issued an average of $63.9bn-worth per month, net of repayments.
During the same period in 2017 the monthly figure was just $19.6bn. As banks buy Treasuries, their cash piles fall.