Sweets and acidic foods, such as candy and sugary sodas, can cling to teeth and cause caries. Oral cancer and gum problems can be caused by smoking and/or chewing tobacco products.
Although your molars are robust enough to crush ice and rip open packaging, doing so might cause them to crack and put undue strain on your jaw. When you're worried, gnashing or grinding your teeth might cause them to crack.
Another unhealthy habit is biting your nails. It drags your jaw out of place and alters the fit of your teeth.