What reasons are there for supervised child visitation?

Just curious, I saw this today at lunch a child visiting his mother with a social services person supervising it all. I was wondering what exactly do you have to do wrong as a mother to warrant supervised child visitation?? Is this a fairly common thing, or do you have to have done something really bad?  I assume, violence, drugs/alcohol or attempting to kidnap them, are a given, but are there other reasons?