how its possible that warrior throw a archmage vargoth at turn 8 with 8 mana, after end of turn with 4 mana left vargoth used a spell for 7 mana while warrior didnt?
Which spell? character
Warrior with 8 mana
First: Archmage Vargoth (4 mana left)
end of turn
Vargoth used spell Commencement (for 7 mana)
( Summon a minion from your deck. Give it Taunt and Divine Shield .)
You should probably post the replay on that, only way I can see that happening is if they had a Kael’thas on board, played some small spells into a discounted Commencement and got lucky with Vargoth (Or got Vargoth from the Commencement)
My guess is that you weren’t fully paying attention and they got Vargoth off a played Commencement, which then cast a copy at the end of the turn.
Could be wrong but without a replay I’m always gona assume human error over a bug as the first option, even if it’s myself 
Im healthy yet and not blind. There wasnt Kael’thas and the table was totally empty at all well nevermind, still funny and weird