What is the first sign a client relationship is quietly going wrong, before they complain?

I have started noticing that clients rarely announce when something is going wrong. They just get slower to reply, or their questions get shorter, or they stop asking about the thing they used to ask about every week. By the time someone actually complains, the real damage happened weeks earlier and everyone just called it a busy season. I am curious what the earliest tell has been for other founders, the one signal that in hindsight always shows up before the complaint does. What is yours, and did you catch it in time or only see it afterward?