What are the most common mistakes first-time entrepreneurs make?

Focus on maximizing short-term profits by targeting the wrong customer/market segment.

1. They spread too thin , too fast.
2. They jump the gun and start too big, (keep it simple).
3. They don't have a long term vision.
4. They stall too long, if you have an idea act on it, talk to people, learn, (Fail Fast).
5. They put all their eggs in one basket.

They ramp up their headcount too fast and bleed their capital away in monthly salary overheads - I've seen this happen to so many people.