My mod needs to modify the "renderAttackIndicator" method form guiIngame. So I created a new class "BCGuiIngame" that extends GuiIngame and modified that method a bit. Then I use my gui instead of the vanilla by doing this

    
@SubscribeEvent
public void onLogin(PlayerLoggedInEvent event)
{
	Minecraft.getMinecraft().ingameGUI = new BCGuiIngame(Minecraft.getMinecraft());
}

 

The problem is that this makes other mods that use the guiIngame crash, so can I do?

PlayerLoggedInEvent is server only event.

Minecraft.class is client only class.

 

http://mcforge.readthedocs.io/en/latest/concepts/sides/

 

You should NOT replace GuiIngame.

 

Use RenderGameOverlayEvent.Pre - to cancel rendering of whatever you want.

Post - to render your stuff.

 

Use ElementType to pick rendering phase (otherwise you render multiple times).