Thomas Shey
Legend
Lack of communication. Not responding to (or even reading) emails or text messages, not updating the schedule calendar, cancelling at the last minute because you "just remembered" you had plans...and then complaining about how you are "totally lost" and "can't follow the plot" (if you're a player) or about how "nobody is taking it seriously anymore" and you "need more engagement" (if you're the DM).
If we communicate, we can handle just about any problem that comes up as a group. But if you don't/can't/won't communicate, you are the problem.
This is all fair, but in its milder forms is so common it probably isn't really useful when separating the genuine bad.