There are so many of them.. Frankly i myself committed most of these mistakes during my newbie days.
1. Thinking that you have learnt everything: Most of the new traders start with their basic trading knowledge and towards the end they feel 'I am a pro now, i know everything.'
2. lack of regular practice: Even if you are doing good in your trades it doesn't mean you will be in profit whenever you trade. regular practice is the key to gain experience. Without experience one clearly doesn't know how uncertain and volatile market can be.
3. Greediness of course: The day you start trading to hound the profits and nothing else, you will start losing your capital. Greed would make you place larger trade size which can certainly flip your account.
4. Taking too much risk: Yes, taking risk is required when it comes to trading, but taking risk beyond your trading capacity is unethical.
There are many more common mistakes, but apparently these effect your trading the most.
The more your practice, the more you learn.