Documentary mistakes

things i learned:

• that a soundtrack could make or break a documentary 
• that you should do script after shooting in a documentary
• that documentary writers sometimes don't even have a strong vision in mind

opinions:

• i like how this tells you all the simple mistakes you could make
• i don't like how some mistakes that can occur could be avoided with a little common sense so why point them out

question: why don't they make sure the have a strong vision before they go with the idea?