Some astrologers have the belief that when we strengthen planets through the practice of remedial measures like mantras, puja (worship), yagya (fire ceremony), charities etc. that the planets, especially malefic planets, can cause more harm. For instance, if someone is going through a difficult dasha and/or transit of Saturn that they shouldn't recite a Saturn mantra.
This is not a traditional viewpoint. In the classic text, the Brihat Parashara Hora Shastra, sage Parashara says:
"At the time when any planet is turning inauspicious for the native, the said planet should be propitiated with proper worship and oblations (offerings) because lord Brahma has given a boon to the planets that whosoever would worship them, they should do good to him.The growth and downfall of mankind, the existence and destruction of the world are solely dependent upon them. Therefore, the planets are the supreme amongst the divine gods." Ch 86, 26-27.
I recently asked a teacher of mine, Swami Satyananda Saraswati, who is a priest at the Devi Mandir in Northen California, about this question of whether propitiating planets can cause harm. He said:
"We take the opposing view. Each of the planets has both a positive and negative character. By reciting the mantra of the planets in question, we strengthen the positive qualities and endear the general characteristics so they are more propitious to us, the worshippers. The negative qualities are reduced.