To narrow down Cheri Thomas’ answer: failing to scope down your thesis as you go. You are always more ambitious at the start of the thesis than you need to be, and you will need to say less than you thought you would.
Another is that they set too high a standard for their dissertation topic. The dissertation is something to get over and done with. It’s your first piece as as academic, not your greatest piece.
Now, for all too many, it will be their only piece, unless they’re happy signing up to a lifetime of being a TA and penury. But it’s still true: as a fellow student once said to me, “It doesn’t have to be good, it just has to be done.”
Not having a plan and methodology and a strategy (which, inter alia, will help you scope it down when you need to). If your supervisor is doing their job, they will help you have a plan. All too often, they don’t; you may need to draw on your peers for that. A PhD is a three year project. It is your baptism of fire in project management. That is something you will not have learned from undergrad.
Unless you’re one of those horrible ghastly geniuses that can pull off a dissertation in 10 pages, in one of those disciplines where you can get away with it, it’s a mistake to assume that doing a PhD is about being brilliant. It isn’t. Brilliance gets you started. Slog and persistence get you to finish it. The slog is not fun (though the peers are). The motivation is even harder, especially if you’re part time.
Oh, doing a PhD part time? Especially if you have family responsibilities or teaching responsibilities? Feasible, but much, much, much harder. Much more of a requirement that you have disciplined project management about it. Not being brilliant was not a predictor of failing to complete. Not having enough time in the week to focus on the project, I’m afraid, was.
Oh, and if you want an academic job at the end of it, which is a slightly different topic, there are some other fatal errors, which I can certainly attest to:
- Not doing something fashionable
- Not networking
- Not publishing early and often
- Doing too much TA work