Missing stair is a term coined by blogger Cliff Pervocracy in 2012 to describe an abuser or harasser who is tolerated in a community, even though their misdeeds are well known. The analogy is that of a missing step is in a staircase, that everyone in the house avoids and generally tries to warn others about, but which no one actually assumes the responsibility for repairing.
I've personally found the pheonomenon to be very common in all sorts of groups though especially so in so-called 'geek subcultures', possibly because of the tendency for these groups to avoid excluding people except as a last resort:
Geek Social Fallacy #1: Ostracizers Are Evil -- GSF1 is one of the most common fallacies, and one of the most deeply held. Many geeks have had horrible, humiliating, and formative experiences with ostracism, and the notion of being on the other side of the transaction is repugnant to them.
In its non-pathological form, GSF1 is benign, and even commendable: it is long past time we all grew up and stopped with the junior high popularity games. However, in its pathological form, GSF1 prevents its carrier from participating in - or tolerating - the exclusion of anyone from anything, be it a party, a comic book store, or a web forum, and no matter how obnoxious, offensive, or aromatic the prospective excludee may be.
While the term was coined in regards to sexual harassment and abuse in communities, it really applies to all sorts of problem people that communites feel the need to 'work around' and can create huge problems for those communities down the line if ignored. Unfortunately dealing with such members is often anything but easy:
It is critical to the narrative of a missing stair that they are blameless, and the target of persecution. They will almost always state that they would have been happy to have changed their behavior if they had only known. They may try to state that they were never informed of wrongdoing, knowing that the organizers will not be able to give proof as they are protecting the privacy of those who came forward. They will complain that the decisionmakers were too harsh, they will state that the community is actually not safe for cis/het/white/males or some combination. They may call you a feminazi. They may tell others not to go to your games because you are aggressive and overzealous in your harassment policy. They will cite their own awesomeness as proof of your persecution. They will position you as hurting the community and position themselves as defenders of it. They will seek attention for the pain you have caused them. They may make accusations about you personally, or claim that you harassed them with your decision. They may make a public spectacle on social media. They may cost you players, money, and mental health. It is their goal to make this so difficult that you will wish you hadn’t taken the decision and that maybe others would think twice before doing so in the future.