There are a number of things that come to mind:

1.  Sitting around the office waiting for a call

2.  Lack of capital to get you through those first few months or even years

3.  Not finding a suitable Mentor

4.  Not being Self-motivated, this is minimally a 9-5 job ... not part time (not if you want to succeed)

5.  Follow up - consistently and promptly (the hungriest bird gets the worm)

6.  Working hard, Being Honest, and Doing what you say you are going to do

I don't think there is just one mistake, but several. Not having a plan, no mentor to show them the ropes, not working hard enough, not having enough assets to live on, not being a "go-getter", and of course, not joining AR. Many could learn from that alone. The job is not easy as people think, so you must put in the effort.

Thinking that real estate is easy money. 

Not having enough money saved first to get them through the first few financially difficult years.

"....in them leaving the industry."

They failed to understand that a real estate agent is  a small business owner. That failure led them to open a business without knowing what their business was. 

If you do not have a business plan defining your real estate business, you will be counted among the 90% of newbies who do not see their 2nd year anniversary. As a result you will leave a pool of money behind and take a pile of debt forward.

ooooh... only one?

How about

* thinking that this is a job that doesn't require any skill?
* thinking that business will come to them?
* not connecting with their sphere of influence?
* not setting aside enough money to survive the 1st year with zero sales.

They think that once you get the real estate license, the phone will start ringing non stop.  

You have to market your self and tell people in your area that you are in business and ready to help with their real estate needs.

Not showing up every day and being consistent.  Chasing the next shiny thing is a killer sometimes.

 I believe it is not having money put away to start with not knowing how expensive real estate can be ...

Steven Ting Under capitalization.

Not understanding the importance of and knowing how to prospect consistently, efficiently, and efficiently.  

Starting with the WRONG Broker/Office !!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!

not having enough cash reserves

They don't work hard enough or smart enough and they are terrible underfunded.

They don't post their photograph and no one takes them serious.

Thinking that it's going to be easy.

My first thought is that they do not realize there will not be that paycheck in that new career field, until possibly months down the road, while in the meantime they are hopefully laying that groundwork to make contacts and get clients, which has to be worked at on a daily basis.  My second thought is that they do not realize that you can work 7 days a week.

Not having enough in savings to sustain the ups and downs between commissions as some others have also indicated. It is hard to budget money in this business and sometime all the fees get overwhelming.

Thank god gas is cheap now.

Not making any money.

They have no awareness of feast or famine something this business runs on

Mostly going into business under capitalized.

Great question and answers.

You got all right answers. 

Not showing up for training and coaching.

Bad Reputation with fellow Realtors.

Buying every new fangled tool possible, not having enough money in the bank to support the mandatory costs and not putting in the time it will take to make it.

Most are financial failures. You have to do what is necessary to generate income.

Not managing their money, not starting with enough working capital, not making a basic business plan, thinking people are just going to find them.

They get 'sucked' into all sorts of programs offered by industry parasites.

Lack of understanding that the real estate licensing process does not teach you how to make money in real estate... 

Cash reserves.

They treat real estate like a job and  not a business.

Thinking it's an easy job and you make your own hours.

Good morning Steven. Not having a mentor or coach and not focused on networking and blogging.

Good business planning is a great start. Salery expectations go hand in hand with marketing budgets.

Not putting their profile pic in AR!  Seriously, it shows a lack of commitment to quality.

Steven Ting This is very good interesting question.

not posting a picture on Active Rain

They don't have a business plan...or they don't have a realistic one.

Also, often related, they don't have a enough liquid capital to get them through the start up phase.

Forgetting that our business is a number's game.