[译] 吸烟是个坏习惯。
[误] Smoking is a bad custom.

[正] Smoking is a bad habit.
[注] custom 指“（社会的）习俗或惯例”，属群体性质的；habit 指“（个人的）习惯”。如：Social customs vary in different countries. 社会习俗因国家不同而产生差异。