Rumours are that the Federal Reserve Bank attempted to reverse its quantitative easing programme last week, or rather attempted to test the mechanism, and failed. This only a rumour mind you.
Supposedly they attempted to sell $100bn worth of assets (TARP assets? Presumably not) under a repo agreement, ie agreeing to repurchase them later for more, so no risk to the buyers - the "for more" bit giving the buyers some interest - and the bond buyers sat on their hands and refused to bite.
Had the operation worked it would have withdrawn $100bn of liquidity from the US markets for the period of the repo agreement. (The Fed aren't so bold as to attempt to outright sell the assets; probably wise, that could trigger a price collapse.)
Back to the drawing board, guys!