GOD is like the wish-yielding tree of the celestial world (Kalpataru), which gives whatever one asks of it.
So, one should be careful about what one asks from God because of accrued good karmas. Ramakrishna Paramhansa explains this through a beautiful story.
Let us listen to the story: A certain traveller came to a large plain in the course of his travels. As he had been walking in the sun for many hours, he was thoroughly exhausted and heavily perspiring. As a result he sat down in the shade of a tree to rest a little.
Presently he began to think what a comfort it would be if he could but get a soft bed there to sleep on. He was not aware that he was sitting under the celestial tree. As soon as the above thought rose in his mind, he found a nice bed by his side. He felt much astonished, but all the same stretched himself on it.
Now he thought to himself, how pleasant it would be, were a young damsel to come there and gently massage his legs. No sooner did the thought arise in his mind than he found a young damsel sitting at his feet and massaging his legs.
The traveller felt supremely happy. After some time he felt hungry and thought: “I have got whatever I have wished for; could I not then get some food?” Instantly he found various kinds of delicious food spread before him. He at once fell to eating, and having helped himself to his heart’s content, stretched himself again on his bed.
He now began to revolve in his mind the events of the day. While thus occupied, he thought: “If a tiger should attack me all of a sudden!” In an instant a large tiger jumped on him and broke his neck and began to drink his blood. In this way the traveller lost his life.
Such is the fate of men in general. If during your meditation you pray for men or money or worldly honours, your desires will no doubt be satisfied to some extent; but, mind you, there is the dread of the tiger behind the gifts you get. Those tigers— disease, bereavements, loss of honour and wealth etc.,—are a thousand times more terrible than the live tiger.
Source: Tales and Parables of Sri Ramakrishna