In my view, "master" is someone with some "mastery" in an art or tradition to pass on and pass down ... from carpentry to medicine to martial arts to Zen Buddhary. It need not mean the "master" is perfect, one can be a "master carpenter", yet not every corner will always be smooth; a "master surgeon" cannot cure every patient, and even the most gifted lifesaver of thousands may sometimes make a bad cut or deadly error. However, one should be pretty darn skilled in applying the art in life, and much more skilled and competent than those without the skills required. As in mastery in the martial arts, there is no technique for never being hit or never losing one's footing ... let alone for winning every battle ... there is no training offered on how to never fall, but rather, endless training on how to fall well. Show me the man or woman who encounters life's obstacles, sunny and rainy days, loops and losses, ups and downs ... all the mess and mayhem of Samsara ... who may be sometimes knocked sideways or down ... but who demonstrates how to be hit well and recover one's footing ... and I will show you a great Zen teacher.
One of the unfortunate aspects of religion is the tendency to put the leader or "guru" on a pedestal as being perfect, beyond any and all human failing ... always wise, never saying the wrong thing, always balanced and in control. The Lineage legends and Sutra story books tend to dip in gold and place on pedestals all our long dead ancestors, scrubbing them of every human failing. I think that unfortunate. Plenty of wannabe cult leaders are ready to play to such an image even now ... and plenty of "need a daddy to tell me what to do" students are ever willing to buy into it. That is a shame.
In fact, there are really no "Zen Teachers" ... for Zen cannot be taught. The "Teachers" are more like experienced "Dharma Friends" offering tips and coaching to help the seeker do all the heavy lifting ... and sitting ... on their own.