Hands down the biggest mistake:

Not finding a customer/client who is willing to buy your product/service before you ever launch it.

I've seen many startups tank because once they launched, they finally found out no one would buy their service or product..

Being too reserved and waiting for "the right idea" before releasing a product and running with it

1. False assumptions, and any activities that lead to them. (Listen to your users/customers)
2. Burning capital faster than you can make it without one helluva plan or revenue stream in place.
3. Borrowing too much outside capital and losing control of the business (puppet strings), Felix Dennis would be proud.

Just a heads up, I'm still in the pre-corporate entity startup phase. I've been fortunate enough to have the time to learn as much as I can about the process in the last year or so as possible (while working/writing).

Choosing a niche that is not big enough.

If you get this one wrong then a domino effect follows. I repeatedly see entrepreneurs picking niche's that are too small or saturated. Doing your market research upfront and picking the right niche can be the difference between a success and failure.

Cash flow management is the biggest mistake because you can't recover easily from it. I personally know some great guys who could be successful entrepreneurs but will never be successful because they spend too much money in their business.

Another big mistake is hiring expensive people or consultants and expecting that they can magically solve your problems. Often they just add to your costs. Much better to hire people on a results basis wherever possible.

And yet another mistake is thinking that your "big idea" is really great without testing it in the market place. Some companies spend years developing a product and then launch it and it's a total flop. Customers don't want it!

Successful entepreneurs don't work in their business, they work ON their business.

I strongly recommend that anyone thinking of starting a company read "The E-Myth". The author points out that most people who start their own business think they are entepreneurs (people who are good at creating a business) when they are actually technicians (people who are good at doing a specific job). As a result, they don't create a business, they create a job that consumes all their time and that basically owns them.

Successful entepreneurs understand how to structure a business so they DON'T have to be there 24/7 in order for the business to work. The ideal is to create a business that runs itself, so you have to put in the time to figure out the org structure and processes that will enable this. At startup the entepreneur is wearing all of the hats, but must learn to hire the right people to fill in the org chart, and who will follow the processes (and help to improve those processes) so that what needs to happen happens. If you can't make the leap from technician to entepreneur, then your business will eventually fail if for no other reason than you get tired of working 80-hour weeks and go back to working for someone else.

A well-run business should not be in crisis mode, shouldn't require a lot of fire-fighting, and should allow the owner to take considerable time off if desired. Otherwise, you don't own the company, the company owns you. How fun is that?

The biggest mistake of any entrepreneur is when he becomes complacent and assume that his under commands will take care of his business and he shows lack of attention and interest in putting extra efforts to make business successful that is when his downfall starts. His profit declines and as a fallout he lose money.

Not to focus on one thing

Assuming that they have ALL the necessary skills to run a startup and build a succesful business.

I must say giving up way too early many of them think you will come in and make millions right away however it does not happen right away and they give up.

i guess the one that you learn most from and never forget

Choosing to be an entrepreneur ;)

Speaking their mind.

Allowing expenses to lead revenue.

Not knowing when to stop. When to cease and desist.