A little knowledge is a dangerous thing as if we don't know completely about anything will cause harm to us it can be in matter of anything.
There should be a complete knowledge about any thing else it will not be good. If its a case of study and we know don't know completely we will lose marks. In the same way if its related to any passion like operation if doctor won't know completely a large number of patient will die.
That's why it is said that a little knowledge is dangerous it doesn't what case it is.