These are three I've made or seen:
1) Not doing enough customer development. A lot of first-time founders are scared to get out of the building, or even get on the phone, and talk with potential customers. They don't ask enough questions, and they don't learn to look for not just what people will say, but what they're actually doing. Don't spend 6 months working on a product to find out no one wants it (or wants it the way you've built it).
Start with customer conversations, build mockups, have conversations about the mockups, build prototypes, watch them use it. Listen, build, listen, build, watch, listen, build, repeat. Quick, iterative cycles driven by customer needs are how startups succeed.
2) Bringing friends on to the team that can't really contribute. Startups are the team. Not the idea or the market, but the team that's executing. You want cofounders and early team members that you work well with, but sometimes you make the mistake of bringing someone on board more because you'd like them to be able to contribute than because they actually can. It doesn't end well, either for the startup or the friendship.
3) Not moving fast enough. As soon as you start, you're on a countdown to death. Money's running out. Opportunity windows are being close by competition and market changes. The longer you go without boosts to morale, the more likely it's going to go down. Getting customers is the only thing that adds significantly to that clock.
Avoid time traps. Don't get lost in the big picture forest. Don't get stuck in detail ditches. There are many little things it can make a big difference to spend some time on in a startup, but if the majority of your time isn't being spent on building and selling (which customer development is a part of), you're making a big mistake.