In the case of governments, the future cash flows come from at least two sources: taxes or new borrowing. Taxes can be used to discharge the debt fully, but new borrowing would only change the structure of the debt, not discharge the debt. A may be paid back, but now B would be indebted to C. There's nothing inherently wrong with rolling over the debt in such a manner, provided C is willing to lend B money.
Now, if the government is the sole money producer for a nation, a third method of paying back debt is available: seignorage. Seignorage is the process of printing (physically or electronically) new money (which is zero-maturity debt in this case) for revenue purposes. Note that printing money for circulation purposes is not the same as seignorage.
Let us suppose we are at the point that B needs to pay back A, and B is not the money monopolist. If B has the free cash flow from taxes, no problem: B pays back A and the debt is discharged. If B hasn't the cash flow from taxes, but can roll over the debt (either with A or with a new creditor C) still no problem. However, if B is distressed and can't roll over the debt then B is in default and needs to restructure the debt. This typically has long-term consequences for B's growth, and so is rightly feared.
But, what if B is the money monopolist? Well then B can simply print up all the zero-maturity debt needed to discharge the debt owed to A. No need to worry about taxes or finding another creditor. What are the consequences of this action? At the time of debt maturity, B is swapping zero-maturity debt with zero-maturity debt. So it might look like the net effect is nothing. However, A was expecting to be paid dollars of a certain value - dollars from the current stock of money that has a certain purchasing power. But B has not done so - B has increased the stock of money by the amount of the debt owed to A and thus has reduced the purchasing power of money. A has been made worse off than expected because of this. We see also that the debt itself has not been discharged. Rather, the debt has been restructured in just the same way as issuing new bonds. The only difference is that the maturity structure of debt has been shortened, rather than maintained. That moves claims to real assets closer to the present and this is the source of price inflation.
If B does this once or twice, for relatively small amounts, the negative consequences are likely to be minimal. However if B does this frequently, present claims will come to dominate the maturity structure of the debt and one will observe quite a decline in the purchasing power of money. Furthermore, this will hamper B's ability to borrow since the As of the world do not want to lose money on their investments in real terms. Thus the political consequences of, say, the Weimar republic.