We used to call them “contraindicated exercises”, but the fitness industry has gone politically correct, and we now call them “high risk” exercises. Whatever the name, these are exercises most people should avoid. The one I see most often is the unsupported forward bend, also known as toe touches.
This position places a great deal of stress on the spine, including compression and shear loads, which will eventually cause injury. I believe this exercise remains popular because it feels good temporarily, but if done repetitively, you will end up with an injured back.
As you can see from the picture above this person is leaning forward with a very rounded back, so he is mostly hanging from his ligaments.
You can perform forward bends in a much safer manner (as in Yoga or Pilates), by bending your knees, keeping a neutral spine and executing a hip hinge. This means you are going forward but keeping the normal arch of your back, the bend is in the hips, not the spine. You can also limit the range of motion by placing a block or low table in front of you. Notice how the woman in the first picture below has her hands on a yoga block; this is great for those who have less than flexible hamstrings. In the second picture, she has her hands on the floor, but she is flexible enough to do so without overly rounding her back.
Another option in the picture below is to stand with one leg bent, bring the other forward slightly and straighten it. Keep your spine neutral (maintain the normal arch of your back), and hinge forward from the hip, placing your hands on the bent leg for support.
If your goal is to stretch your hamstrings, the safest stretch is on your back with one leg straight in the air. The other can be bent or straight on the ground, hold the straight leg with both hands, and pull it close until you feel the stretch in the back of your leg. And of course, make sure the stretches are done after your workout (post from September 15)!