Devotee: (translating) He wants to know if the greatest offense is to disobey the guru.

Prabhupada: Yes, that is the first offense. Guror avajna, sruti-sastra-nindanam. Sruti-sastra-nindanam guror avajna. If you accept guru and again disobey him, then what is your position? You are not a gentleman. You promise before guru, before Krsna, before fire, that “I shall obey your order; I shall execute this,” and again you do not do this. Then you are not even a gentleman, what to speak about devotee. This is common sense.

From Srila Prabhupada’s lecture on Bhagavad-gita 2.11 — Mexico, February 11, 1975