There are a lot of bad habits. Smoking, doing drugs are the most harmful for man's health, but there is a bunch of some habits that aren't very good.
For example, one of the most popular bad habits is eating too much sweets. Sometimes people eat a lot of candys, biscuits, cookies, chocolate, especially if they are feel sad or depressed. And in mostly cases they just can't keep it under control. Moreover, the weight of the body starts rising and appear many skin problems.
Also, there are habits that can have a bad influence on your relationships with other people. For example, a lot of people often interrupt their friends or colleagues while talking. This can hurt them. Everyone should be careful and attentive for each others.