A missing stair is a metaphor for someone in a subculture, group or industry that people in the know have learnt to avoid but new people moving through the space may fall afoul of. Systemic barriers prevent people from talking openly about the problem person....but without systemic processes to warn or remove the person, new people will fall afoul of the problem. There is also, I now realise, the Entire Missing Staircase. The person is not just a problem, they attract a locus of people/problems, or consume a whole wing or chunk of the culture.
Some people who go down the hallway will trip and fall and wipe out of the whole hobby because things are so toxic. Meanwhile, a bunch of people live in and around the staircase and don't understand why people don't get their missing staircase vibe (which, they assure you, is really cool/chill). You can't criticise the staircase being broken from the inside, because they like it that way. You can't criticise from the outside because the staircase has become a supporting wall of the whole subculture and you can't pull the whole house down.
And they keep attracting people who don't mind climbing up the walls, which ends up making the subculture weird or cliquey because you have to nail your colours to a mast instead of just all mixing together, or people who think the whole house is falling down just don't come in at all.
And maybe the house ends up not talking to each other, or everyone packs into the broken staircase and pretends it works.
If you're lucky, the staircase falls down eventually, and hopefully doesn't take the whole house with it, but it sure does a lot of damage as it falls.
