WordNet sense 1 (having the attributes of man as opposed to e.g. divine beings):

frail, imperfect

WordNet sense 2 (having human form or attributes as opposed to those of animals or divine beings):

human

WordNet sense 3 (likely to fail under stress or pressure):

weak

WordNet sense 4 (liable to be erroneous or misleading):

unreliable

WordNet sense 5 (straying from the right course or from accepted standards):

errant