If the rendering is not done 'racing the beam', it will show bad artifacts.
Just like in case of v-sync in which you see wrong frame, now you would see the frame during it's construction.
This means that you see whatever was finished at that point of time when image was read to the screen, you rarely see finished image.
IE.
First refresh background was finished, but characters were not started, next refresh you have only rendered part of the background.. etc.
Amiga workbench came with some rocket demo that did it like this and it was not very pleasant to look at.