Socializing can be prompted even when you're working on another project, and breaks the project (it will not be worked on when socializing is done, and is locked as being worked on so it can't be reassigned, only canceled).
When a girl goes on hiatus, the game does not tell you that it will do whatever it wants to the roster. It does, however, do that and reslots whatever it feels like to make up the space for the missing girl. This should not be done, because you can still schedule girls on hiatus for events and just have to delay it until they're back, there is no reason it should retroactively change things already established and even less than no reason for it to do so without telling you. Caused 3 disasters in my concert thanks to that.