What common pitfalls to you believe new leaders can often fall into? How can they avoid these mistakes?

Some of the pitfalls for new leaders are : 

1) Failure to learn & adopt the existing Organisational Environment/Culture and Systems, Processes & Procedures.

2) Promoting the culture of Sycophancy & Boss-appeasement and banking upon them for Decision Making instead of adopting to the Management Plans, Processes & MIS etc.

3) High ego, arrogant behaviour, poor Stakeholder Management w.r.t. employees, sponsors (owners), vendors, associates, customers, govt bodies etc. can result in resentment, non-cooperation, and opposition.  

To avoid these pitfalls, new leaders need to keep learning, be adoptive, be flexible, follow systems (be system oriented), have patience, be good and polite to all and develop good relations with one and all.   

Enforcing learning without setting examples;

Suppressing and not allowing others to make their opinions.

I somehow do not concur with the logic of the question .. Furthermore,  the term "new leader"and the implication that this new leader will be easily fooled by pitfalls and fall into mistakes ... is somehow above the perception of what a leader should be and what he/should be doing, and who (I wonder) would be higher than him in the organization to judge him whether he made mistakes or not !!

In my view, a leader is NOT appointed, but enforced by the acceptance of the followers who buy into his vision and follow the direction he/she is setting to fulfill that vision

Agreed to Mr. Ghani with an addition, to that is, avoiding mistakes there are only two ways, one is to gain help of experienced person and second is to have mistakes done and avoid to repeat them