On an island with only 2 people and one central bank, both A and B loaned 1 million dollar at the start of the day, the loan have 0 interest but must be returned at day end
A paid B 1 million dollar for B to sing a song, and B paied A 1 million dollar for him to dance in the court
A and B will be able to return the loan at day end. We could say that both A and B were doing stupid things but as long as they are doing it together, there will be no problem
What if only A paid 1 million for entertainment but B do not order 1 million entertainment from A? Then A will not be able to return the loan and B will have some saving, and B think A is a stupid guy: paying 1 million for a song
It's a typical "race of thrift", if one of them can live a low standard life by spending much less than the other, he will accumulate money and put the other into debt
Money is a tool for cooperation. If B can not buy anything to A then the money he owns worths nothing. So there is no problem.
In reality, since there are billions of people, even B won't buy equivalent worth of products from A, A might still be able to sell something to C, so the reduction in consumption from B will not be felt directly. But anyway, increased spending from C will require increased sale from C, thus B has to buy something from C to keep the whole system in balance
Besides, both A and B usually have high incentive to hold a little extra money, because they regard money as "wealth", they even want to give this "wealth" to their children and grandchildren...
Therefore, money supply does not only need to fulfill the requirement for transaction, it also need to fulfill the requirement for saving, that's the reason in a financial crisis, the money supply must increase dramatically, since lot's of people's behavior changed, they started to save more and spend less