I had experienced that bug many years ago as well. It's easily reproducible, as in here:

Click to reveal..
Code:
///////////////////////////////
#include <acknex.h>
#include <default.c>
///////////////////////////////

void main()
{
	fps_max = 30;
	video_mode = 10;
	level_load(NULL);
	camera.arc = 40;
	camera.y = -512;
	camera.z = 128;
	camera.pan = 90;
	camera.tilt = -15;
	you = ent_create(CUBE_MDL,nullvector,NULL);
	your.scale_y = your.scale_x = 64;
	c_setminmax(you);
	set(you,POLYGON);
	me = ent_create(CUBE_MDL,vector(0,0,60),NULL);
	set(my,SHADOW);
	def_move();
	while(1)
	{
		draw_text("Hold [Space] to move the entity.",20,20,COLOR_RED);
		my.skill1 += key_space*10*time_step;
		var x = sinv(my.skill1);
		my.x = pow(abs(x),0.5)*sign(x)*512;
		if(key_ctrl) reset(my,SHADOW);
		else set(my,SHADOW);
		wait(1);
	}
}



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