QE happens when central banks buy (crap) bonds from financial companies, giving them liquidity to buy better assets. Consequently boosting the stock market as we have seen.
But what happens if these bonds bought by the central banks default?
For the US:
Quantitative easing was the monetary policy where the central bank purchased government bonds in order to increase the money supply and encourage lending and investment. It lowered the cost of money and increased the money supply by purchasing assets with newly created bank reserves in order to provide banks with more liquidity.
The Troubled Asset Relief Program (TARP) was a law passed by Congress that enabled the US Treasury to buy troubled company assets and stock (toxic assets) in order to stabilize the financial system, restore economic growth, and mitigate foreclosures caused by the 2008 global financial crisis.
Quantitative easing is about essentially printing money out of thin air.
The whole (theoretical at least) point of this exercise was to boost the economy by injecting fresh funds and liquidity to the country's economy via commercial banks and intermediate institutions that were the beneficiaries of the QE policy.
Should the bonds or other financial instruments purchased with those money default or not, should make little difference to the central bank at the end. Also, keep in mind that the primary instruments purchased during such efforts are sovereign bonds (not exclusively though), so if those ever come to default, the implications would be far greater for everyone, the central bank of the said sovereign country included.