The missing stair is a metaphor for a person within a social group or organization who many people know is untrustworthy or otherwise has to be “managed,” but around whom the group chooses to work by discreetly warning newcomers of their behavior, rather than address the person and their behavior openly. The “missing stair” in the metaphor refers to a dangerous structural fault, such as a missing step in a staircase; a fault that people may become used to and quietly accepting of, that is not openly signposted or fixed, and that newcomers to a group or organization are warned about discreetly.
https://en.wikipedia.org/wiki/Missing_stair