If it happens, a DMA underflow will occur causing a tear. For the frame being rendered when this happens, the data under the spot of the display where the tear happen will be misaligned. Because the frame is being updated at a high rate (60Hz?), you see a quick flicker on the display. If the tear happens a lot, the flicker can be annoying.
It should be noted that the LCD controller will underflow only when it's DMA request is stalled for a considerable amount of time. The LCD controller has it's own buffer memory filled up by DMA and can handle short stalls during it's DMA request.
