Have these in mind when you are starting out:
- Single Founder
- Bad Location
- Marginal Niche
- Derivative Idea
- Obstinacy
- Hiring Bad Programmers
- Choosing the Wrong Platform
- Slowness in Launching
- Launching Too Early
- Having No Specific User in Mind
- Raising Too Little Money
- Spending Too Much
- Raising Too Much Money
- Poor Investor Management
- Sacrificing Users to (Supposed) Profit
- Not Wanting to Get Your Hands Dirty
- Fights Between Founders
- A Half-Hearted Effort
Source: http://www.paulgraham.com/startupmistakes.html