If ur spells of a specific school of magic gets blocked (e.g. from Mages Counterspell) ur still able to cast ur Instant Spell Procs.
It works with procs of the talent "From Darkness comes Light" for "Mind-Spike"
or with "Divine Insight" where ur "Mind Blast" gets instant Cast. (And probably with all other similar talents of all classes)
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In the UI ur spell gets displayed with Cooldown on it, just cast it anyways it will work.
This may is just some UI bug but therefore it looks like its not intended.
Tested with priest vs. the dinos on isle of Giants.