One of the strange but common occurrences in producing large pieces of writing is that the writer frequently becomes so close to the text that small (and even some large) errors get completely unnoticed. Supervisors have different ways of dealing with this. Normally I give a detailed commentary chapter by chapter, and then quickly read a revised version, but do not revisit that unless a later chapter forces some sort of re-think. It is usually emphasised from the very start of a PhD that the research project should belong to the student, not to their supervisors, and as the final draft of the dissertation approaches completion, this is a crucial time for the student to assert their ownership. In defending the thesis at a viva, it is the student who will be held responsible for any errors and misspellings, but the supervisors can effectively support this process by timely guidance.
Firstly, in addition to re-reading every chapter as it is drafted, I encourage students to review and revise the entire dissertation just before they start to write the final chapter that brings everything together. In this way writers can check for any small typos and at the same time refresh their memory about what they have written earlier. (It can be a relatively long time between the start and the end of the writing process, and memory can play tricks!) Next, it is usually a good idea to get an extra person (apart from the writer and the supervisor) to read through a document (in stages) to give some feedback. Although it helps to have someone who is knowledgeable about the subject material, the main thing is to have someone that can be trusted to tell you the hard truth. A friend or partner can be a great source of guidance to clarify writing (“what exactly do you mean by this sentence?”). Thirdly, it is a good idea to re-read the dissertation (again!) after you think that it is finished – perhaps not every single page, but certainly to dip into sections at random and check that the detail still makes sense. Do not skim over the small things such as tables or the caption of diagrams, these are just as likely to contain errors as any other paragraph.
It seems superfluous to say, but as each section or chapter is backed up for security, it is important that each saved copy has a date and/or version control number on every page. With multiple back-ups and multiple versions of revised copies, it can be very easy to create confusion. Ultimately, however, there comes a time to stop tinkering or tweaking the text and let it stand on its own merit. In some universities, the submission of the dissertation requires to be countersigned by the supervisor to agree that it is now in a fit state to be sent to an External Examiner for evaluation, but in other institutions the supervisors are simply informed. Either way, the student is responsible for the final contents and its appearance, and the supervisor is responsible for helping the student to produce the best submission under the prevailing circumstances.