Do not pull your neck with your hands this will cause neck pains, make sure to touch it only lightly. Also, avoid touching your chest with your chin.
Make sure to maintain a natural spine. As most crunches, this exercise puts some pressure on your back, be careful if you suffer from lower back pain.
Crunches are pretty good strengthening your abs, but you might throw your back out in the process.
Crunches, or worse, sit ups, involve flexing your spine repeatedly, which get worse if you add load to it, like a weight plate.
To make stuff worse this exercise combines rotation of the spine as well.
A combination of flexion and rotation are the exact mechanism that causes injuries like a herniated disc, and it is the most injurious movement for the spine better to be avoided all together.
To make it a bit less dangerous, try to hold your spine flat on the floor as you bend.