Kassadin's ultimate

When it is on 800 mana and you use it approx. 200 millisecond before the 800 mana cap (that 15 second cd) it takes 800 mana but deals dmg as it was on 50 mana cap. In addition it goes to 100 mana cap except 800 (when you use 800 mana ultimate it goes on 800...). I quite main this champion and I have met with this issue like 30 times or so, sometimes this can change how team fight ends... My play-colleagues have also encountered this issue. One said : " It is quite known issue."