Stretching is one of those things that people tend to not think about much. They simple just go through the motions trying to look as flexible as they can. What ends up happening is they end up moving through the wrong areas and not stretching the muscles that they are supposed to be stretching. This is called "relative flexibility". It is when the body goes through a range of motion and chooses the path of least Resistance. I simply call it "fake stretching". Here are three areas of the body that people typically use "fake stretching":
Hamstrings - The picture on the left is an example of fake stretching for the hamstrings. The picture on the right is correct way to stretch the hamstrings. The hamstrings don't attach to the lumbar spine so flexing over with not increase the stretch. Instead, focus on driving the tailbone back, hinge at the hips and maintain a tall spine.
Hip flexor - This is so common with hip flexor stretching. Notice the large sway in the back in the picture on the left. This is an anterior tilt of the pelvis. The pelvis should actually move posterior like the picture on the right. Want to feel more stretch? Squeeze the glute of the same side being stretched.
Piriformis - Again, the picture on the left is wrong and the picture on the right is the correct way to stretch the piriformis. If you haven't seen this stretch, you can read about it and see the video here. This is pretty much the only piriformis stretch we use anymore. It is much like the hamstring stretch. Don't round your back. Maintain an arch in the low back and simply kick your hip out to the side.