Start small and make your big mistakes early. 

Unfortunately, many make the mistake of solely looking at the numbers when that is not the entire picture.

This is a mistake that novices make all the time.

Sometimes you realize you made a mistake for some reason and decide to make a change. 

I made this same mistake when I went to buy "Sweet" Corn for the dinner I threw last time I was there.  5 ears for $10. 

Most people make the mistake on putting to much moey down or use "All cash" and then need to pull it out later at a higher rate.

I've made this mistake before myself. 

You sound like you might jump at the first bad application that comes in so you need to watch out for that as that is the worst new landlord mistake you can make.

A principal (who is not well versed in contract law) in a transaction can draft their own contract form just like someone accused of murder can represent themselves...and both would be making a grave mistake.

A deterrent for others to make the same mistake.