Hmm.. from what I know... most of the time the trap was either they were staying at a mind state called jhana ,especially Lokiya Jhana (there's five kinds of Rupa Jhana and four kinds of Arupa Jhana which classified as Lokiya Jhana, and another called Lokuttara Jhana, a state of mind that having Nibbana as their object, according to Abhidhamma)... or they may fall in a mind state called bhavanga (the mind didn’t know their object, basically like sleeping knowing nothing and some yogi feel so fresh after wake up from this state of mind)in Buddhism terms.

First jhana mind state actually isn’t that difficult to reach especially for those who possess a natural calm mind. But the challenge is staying at that state for as long as possible and mastering them.

Some guru ask their yogi try to stay in that state for one or even five hours. After that, the guru will teach them how to master each level jhana, called jhana vasi, and then using this state of mind as a tool to see Paramattha Dhamma. The more someone being expert at these state of mind, the easier and the clearer someone can seeing things as they are. :)