I once had a friend whispering to me when I never got the notification he was coming on, he wasn't on my online friends list, and when I clicked to whisper back it told me he was offline. He had to log on and off to fix the problem.
But I'm pretty sure they weren't whispers I'd already gotten. I'm not sure what that was all about. Some sort of weird server glitch, I guess?
The fact that you disco'd afterwards makes me think that something went wrong.