Guild Wars 2

This has happened on many toons on various races/professions.

You cast healing, you see the heal result up your health circle but still you get downed and your heal that had already added health to you disregarded and down we go!

It’s like pulling the rug from under!

I agree, I’ve been taken down a split second after activating heals that would have kept me from going down. I have also had it happen with Mist Form where I’m suposed to be immune to damage but it still hits because the Mist Form hasn’t taken effect yet… but it still acts as though I used it and I have to wait for it to reset to use it again. (I know because I’ve rallied right afterwards.)

This is the result of inherited lag in online games. You always few milliseconds behind the server. The server can not track your actions continuously even your movement, and your game client is built to show smooth movement/actions not to be continuously waiting for server feedback, I am saying continuously here meaning every foot step is to be confirmed by the server.
Now, if you have above normal lag, this behavior is magnified. Check your game latency from Task Manager → Performance → Network ; and see GW2.exe latency.