You're holding a cat. They're just being chill and nice right this second, but that's right this second. You must watch them carefully. Very carefully. Are they looking happy? Are they looking mad? You must listen to them closely, for even the slightest grumble or "mrrp" or sigh. You need to feel their movement — each twitch, or how they shift their weight, could indicate that they're about to really tell you off.
And then they shut their eyes and start purring. How dare you consider putting them down.