This can happen when a peer has disconnected but we haven't yet been able to acquire a lock to the store to clean up the state associated with it.
Zed Server
This crate is what we run at https://collab.zed.dev.
It contains our back-end logic for collaboration, to which we connect from the Zed client via a websocket after authenticating via https://zed.dev, which is a separate repo running on Vercel.