Okay this is also an inquiry.
Why do people get Gravity Error if there are many monsters in the Prontera City? Does it have something to do with the monsters that doesn't exists yet?
Suggestion
I guess we all know that we can summon MVP or non-MVP through BB given to us by the time we log in.
However, this is becoming abusive. Especially someone in non-Renewal earlier I just encountered.
And he said "I'll make you Gravity Error, noob." He does this by summoning MVP in my screen which actually, I don't get any Gravity Error /shy he was trying to annoy me.

And the others just keep on killing them because I know their screen just keep on shaking as well.

Back to the suggestion, I suggest this kind of item should be only used outside the city to prevent other players receiving Gravity Errors who abuses the power of BB in the city especially in Prontera.
Do note that this happens in the castle to if there are many monsters in a room.