1. Rounded back - Doing any bent over exercise puts strain on your back. In order to
protect your back from injury it is incredibly important to keep your back in a
neutral or slightly hyper-extended position. These pictures are showing a deadlift. The top picture is demonstrating a dangerous rounded back posture and the second picture is showing a safe posture.
2. Locked knees - Knees are complex joints with a lot of room for injury. By "locking" your knees you are putting tremendous strain on the ligaments and the other soft components of the knee joint. Whenever you are doing ANY exercise at ANY time (including most stretches) you should focus on keeping your knees slightly bent so that the muscles are actively working to protect your joint. Here is an example of locked knees and slightly bent knees during a bent over row exercise.
3. Shrugged up shoulders - When we are stressed or working really hard, most people have a tendency to tense their upper trapezius muscles so that their shoulder scrunch up to their ears. This causes increased tension in the shoulders and neck which can lead to headaches, and neck and back pain. It is always advised to focus on keeping your shoulders down away from your ears while doing exercises that aren't explicitly targeting the upper traps and neck muscles. Here is an example of the wrong and right way of doing a seated row.
4. Bent wrists - Your wrists consist of 8 little bones that work together to give you such incredible range of motion. This also means that they can be easily injured under heavy loads. When you are doing any pushing motion you should be aware of your wrist position and try to keep it as straight as possible in it's strongest position. Think that you are punching using your two big knuckles.
5. Loose core - "Core" is a big buzz word right now and it's got a right to be. Your core is comprised of many muscles on the front, sides, and back of your torso, and also the diaphram and pelvic floor. They are important for supporting your spine through movements, and if your core is weak or the muscles are imbalanced, it can be the cause of many back-related issues. Even if you have strong, well-trained core muscles it is possible to injure your back if you forget to "engage" them while exercising. Below is an example of the wrong and right way to do a plank.