In Nucleus, there is a feature that allows players to join a full server. To do this, Nucleus uncancels the ClientConnectionEvent.Login event to allow the player in question to join during the FIRST order: the code to do it is here.

However, LP subsequently recancels the event during BEFORE_POST because it noticed that the event was canceled at AFTER_PRE, thus breaking this functionality as Nucleus performs what it needs to between these events (FIRST, which is after AFTER_PRE).

Nucleus issue: NucleusPowered/Nucleus#848