First time

Mana splattering, Mana disruption and Mana Babel. 

These three things were what a mecha core user feared the most. 

In the case of Mana Babel, there was not even a need for anyone to call for a doctor, if a mecha core user fell into Mana Babel then their hearts would stop beating at once even the most skilled doctor wouldn't be able to revive them. 

As for Mana Disruption, there was a chance for a mecha core user to survive but if they were not taken care of properly their conditions might deteriorate and their core may snap in pieces causing them to become a vegetable for their entire life. 

Between these two conditions lies Mana splattering. When it happens, the mecha core user's condition will not be in danger nor will their core snap into pieces but their core will start releasing mana wavelengths in an uncontrolled manner, with these uncontrollable wavelengths it would be impossible for a mecha core user's body to work normally.