Since the quote function here limits quotes to 45 words, I'm just going to bold and italicize the statements to which I'm responding that I'm "quoting":
Giving incorrect paper details and requirements when ordering.
In my experience, these don't really occur very often. More often, (again, just in my own experience), when customers provide incorrect details, it's more often by shortening the length of the assignment to reduce costs. If a project is assigned as "10-15 pages," that means the professor is expecting the amount of depth and detail allowed by a project of that length and not only the depth and detail allowed by a project half that length. I don't know why professors do this, but when they provide detailed descriptions and rubric of what's expected in the project in conjunction with the instructions to submit a project of between 5 and 8 pages, it's often the upper limit that's required just to do everything demanded for the project. That makes my job harder, but usually won't affect your outcome, especially if you order the full 8 pages; but if the assignment is for 8 pages and you tell me "4 pages," that's basically asking for a bad outcome even if I write you the best project possible in those 4 pages.
Leaving out crucial information.
That happens a lot. A few weeks ago, I mentioned one instance where the client emailed me on the due date (and as I was actually in the process of sending her completed project) to tell me that the topic choice -- the specific disease selected for a Nursing project -- required the professor's approval. Another crucial piece of information that this particular client (and others have sometimes) left out was that she had already submitted previous projects to the same professor before she decided to use my services. If she'd have let me know that, I'd have asked to see her writing first, so that I could provide something roughly similar in writing level. Depending on how you choose to use our work (which is entirely your business), this omission could, potentially, cause you a lot of problems on your end and there's absolutely nothing we can do to help you when you send us a frantic email a week after we deliver a perfect project with "Urgent Help Needed!!" in the email subject field because your professor wants to see you to discuss the tremendous difference between your last two projects. If you just clue me in initially, I can help you avoid that problem in the first place.
Another piece of crucial information that customers often fail to share is that the topic they give me is only one topic on a long list of possible topics or (even worse) a totally original topic thought up by the customer rather than an assigned topic. Sometimes, this doesn't matter; but other times, it matters a lot. If you're hiring a writer, you should always leave any permissible choices to whomever is actually writing the project. There's no advantage to picking the topic if you're not going to be doing the writing, simply because your writer might be able to do a much better job for you on one of the other topics on the list. Similarly, when it's a totally original topic, you should let your writer help you come up with a topic, partly because we're very experienced at shaping topic proposals to the availability of high-quality sources. If you just propose a topic that "sounds interesting" to you, it might make it more difficult for your writer to satisfy the professor's expectations. As a general rule, you should present your writer with everything in the assignment but nothing more than whatever's actually in the assignment, and certainly, nothing less. Whenever possible, just let your writer make any and all decisions that your professor intends to allow you to make as the student.
Procrastinating until the last minute to place an order.
Agreed. The customers who get burned the worst are those who don't even start looking for help until they need a big project due in a few days. At best, if you already have a good trustworthy writer, waiting until the last minute always increases the price of a project substantially. At worst, if you don't already have a good trustworthy writer, you're taking a big risk instead of at least limiting your risk by trying out any new writer with a shorter project, first.
Unrealistic expectations.
I'd only agree with this to the extent you don't already know how good your writer is. Once you've previously received good work from a writer, your expectation of receiving exactly what you pay for in the form of work of similar quality on future projects is probably not unrealistic at all. If you're just going by what any writing service represents on its website, always try to test out that service with a smaller project to limit your risk, precisely because many of them absolutely do not provide work that is even remotely consistent with the representations and promises on their websites.