#1 Not reacting fast enough when they identify a good deal

#2 Stop looking for new opportunities. The deal of the lifetime come by once a week and if you stop you will miss it

#3 Not using a knowledgeable advisor or mentor to stop you from making mistakes

#4 Using too much debt. You need to have either sweat equity or equity in a deal or you will not be able to weather the bad times

#5 Not understanding the numbers, Accounting is the language of business and you need to be fluent in it

#6 Not having a model and sticking to it.  Everyone should have a definition of the kind of real estate that they like to own so when they see it they know that they want it

#7 Doing everything yourself.  Rely on the experts. It is unlikely you can be an expert at everything

#8 Becoming emotional especially when you have a loser. Cut your losses and move on to other opportunities

#9 Thinking that risk does not equal return. Or the real estate markets are not efficient. If you think a deal is too good to be true, you are most likely missing something

#10 Assuming the appraisal equals actual value. The appraisal is simply the opinion of one person.  Do not think for a minute that you can sell it for that amount they identify a good deal