Your whisper was not delivered.
Using Chrome on two devices in the same room, with more than one Twitch tab open on each device (Settings, my own offline channel and its chat, a channel I'm watching, the Live Channels You Follow page), sometimes whispers begin to vanish with "Your whisper was not delivered." This goes away if I close some of the tabs. But having multiple tabs open is standard Twitch life.
Also, sometimes I am whispering to a mod something critical and time-sensitive, and detailed, and then the dreaded not delivered message comes. The worst part? The message has vanished and I have to type it again, while singing and playing music.
It's the most frustrating thing about Twitch, for me.

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WindowstoSky commented
I haven't encountered this problem as much lately, and if it was fixed, I want to thank Twitch for fixing it! Or maybe I've just been lucky :)