According to garuda purana and parashara samhita, one who gives even a single syllable like om must be considered as guru. A sin commited towards a guru is the biggest sin.
A disciple must never disobey his guru’s orders. Backbiting of his guru, disobeying guru, overlooking, criticizing of guru is the biggest sin and it doesn’t have any redemption.
One who commits these sins towards his/her guru will be reborn as a strey dog for 100 hundred times. After that, he/she will reborn as a physically handicapped person for many times and finally takes birth as a dangerous rakshasa in a deep forest.