I would say it's worse than that. Writing it down won't even be enough in all cases, because communication is a hard problem. It is very easy to come out of a conversation believing you know what the other person said, or believing that you accurately communicated your thoughts, desires, or plans and just be completely wrong on all counts.
There is a reason lawyers write contracts: it reduces the likelihood of a miscommunication about what each party expects.
There is a reason lawyers write contracts: it reduces the likelihood of a miscommunication about what each party expects.