When Sabrina is signing her name in blood, the drop of blood used continues to move and change sizes through-out the scene. Moving a total of 7 times in the space of 50 seconds.
When Sabrina has her left hand cut so she can write her name in blood, there is blood running out of her hand between her thumb and forefinger. However, there is no blood visible on this hand for the remainder of the scene.
Witches would know "Samhain" is pronounced "saa-wen".