Geshe Tenzin Tsephel (FPMT, ILTK) warned us recently about the causes for losing the monastic vows:
There is something that the ordained need to be very careful about. His Holiness the Dalai Lama often confers lay vows during teachings, initiations, etc., to large crowds. You may think that it is a great opportunity to take lay vows from His Holiness, but if you take lay vows you lose your ordination and become a lay person. You are no longer a monk/nun because you repeated after the master “I am a lay person. Take care of me in that way.”
The causes to losing the vows are four: not knowing, not having respect/interest, lacking diligence, strong afflictions. One can make many mistakes because of not knowing.
For those of us who are recently ordained, this is an easy mistake to make accidentally.