Here is what the 17th-century Tibetan teacher Tsele Natsok Rangdrol wrote about going astray in meditation practice:
(1) Not understanding that the mind-essence is the unity of appearance and emptiness endowed with the supreme of all apsects, the unobstructed interdependence of cause and effect, you slip into focusing on the empty aspect. Acknowledge this fault called “basic straying from the essence of emptiness.”
(2) Similarly, after engaging in meditation, although you may have merely intellectualy understood the meaning of the natural state, experience has not arisen in yourself. Or, again forgetting that which has arisen, the meaning will not be present within your being although you might be able to explain the words to others. That is called “temporary straying from the essence.”
(3) While what is needed at present is the path itself, you desire to attain some other result later on. That is called “basic straying from the path.”
(4) To regard the sustaining of ordinary awareness of your mind as insufficient while you desire a magnificent mind-made meditation and then search for it elsewhere is called “temporary straying from the path.”
(5) When something such as a disturbing emotion aries, not to know how to take its essence as the path and instead to meditate on some other technique according to the lower vehicles is called “basic straying from the remedy.”
(6) Not knowing how to take whatever arises, such as a thought, as the path, but to block off that instance or having to destroy it before resting in meditation is called “deviation into the temporary straying from the remedy.”
(7) Not understanding that the natural state of the mind essence is primordially empty and rootless, and fabricating such thoughts as “It does not possess a self-nature!” or “It is emptiness!” or “It is just temporarily empty!” is called “basic straying into generalized emptiness.”
(8) Thinking, “Formerly I was distracted following after thoughts, but now I am meditating nicely!” and then remaining in the state of perpetuating that thought, or, thinking that you have mindfulness when you do not and so forth is called “temporary straying into generalizing.”
Not recognizing the key point of the natural state and not resolving doubts about how it actually is, you risk straying into these and various other kinds of incorrect, look-alike meditations. Exertion in an incorrect, look-alike meditation, for no matter how long is fruitless. Some people create the causes and conditions for an evil state, such as being reborn as a naga by meditating on shamatha cessation. You must therefore have an unmistaken meditation.
Moreover, some people regard a dull or sluggish state of mind free from thoughts as shamatha. They presume that vipashyana means analyzing with thoughts. They believe that a solid and rigid fixing of the mind is mindfulness and mistake a state of neutral indifference for the natural. They confuse the ordinary mind of a commoner, who has not seen the original face of the natural state, for the innate ordinary mind free from fabrication. They regard clinging to a good samadhi or the mere conditioned bliss of being free from pain as the innate supreme bliss. They mistake the involvement in clinging to apparent objects without having attained the certainty of recognizing the objectless natural state for the unobstructed self-knowing that is free from object and fixation. They confuse the stupidity in which knowing is blocked for being nonconceptual wakefulness and so forth.
In short, all the different types of mistakes, incorrect look-alikes, strayings and deviations, are primarily caused by not having applied oneself fully to the key points of the preliminaries, such as gathering the accumulations and purifying the obscurations. Hence, the defilements of negative karma have not been cleared away. Next, not having treated yourself with the ointment of blessings, your mind is uncured and inflexible. Not having resolved your doubts in the main part of the practice, you have become insensitively caught up in theory and absorbed in words. Finally, not having taken the practice to heart, you have become a person with a dharmic exterior who is neither a practitioner nor a lay person… There are many of that kind in this final end of the dark age… You must therefore exert yourself intelligently and not become like that.
Source: Tsele Natsok Rangdrol, Heart Lamp, p. 33-36.