1) not screening the company during the interview 2) not preparing by doing "warm-up" interviews for roles you won't be too sad about if you don't get 3) not doing enough interviews to get multiple offers for negotiating leverage
For my first point, you'll learn a lot about the company and how they work from the interview process. If the company has dysfunctional elements to its corporate culture, depending on the size, it's quite likely they'll be surfaced here. Not only is this one of the first impressions of you for them, but it's one of the first impressions of them for you. Pay attention to what your gut and intuition tell you here. Every time I've broken this rule, it's bitten me.
For my second point, I think that people often forget that the only way to get better at interviewing is to just do it over and over again. It's a little hard to do that in a synthetic environment because it's hard to trick your brain into behaving as if it is real unless it actually is. But wait, you may say, treating an interview like practice is unethical! But I'd disagree. After all, if you really do well on the interview but you're not chomping at the bit to join, they might be more willing to negotiate and make you an offer that would make you reconsider. But, you likely know (and can force-rank) the roles you'd be most willing and least willing to take if you had the opportunity, so make sure to go through the sequence from least desirable to most desirable so that you have more leverage.
My third point is probably the most crucial. You need the BATNA (best alternative to negotatiated agreement) of accepting something else or staying where you are to be able to negotiate. Otherwise, you have much less leverage. This is part of why timing and interviewing for less desirable roles is preferable, because you are more likely to negotiate with an offer for your more desirable role by the time you get there.