Not doing follow up enough.  That costs me listings.  Working on better follow up.

I have been fortunate, so far.   Lost time on clients that wasted my time, burned me one way or another are the most costly.  I simply do not have the time to waste, it is one of my most precious commodities. 

Spending too much time on people (team members and clients) who were not going anywhere.   I so want to give everyone good service though.   I have also spent FAR too much money on lead generation. 

I should have purchased many more properties for fix up and resale rather than selling them to other investors. 

Not leaving my original brokerage sooner.

Thankfully no mistakes in transactions that cost us anything

I made a mistake on a septic one time...$3500

Showing a condo, then going to a Village Inn to write up the offer.   The stove was included in MLS, but I did not include it in the offer.    Had to buy a stove.   Have never done that again.  

Not being more aggressive when I first started out. Thinking back I see opportunities where I could have done things a little differently and I let it slip though my hands. Now mind you this was a long time ago.

Not really costly, but 20+ or so years ago when I was a new realtor with 3 school age kids, I sold a tenant occupied property to my young first time buyers. Prior to closing the tenant moved out and stole the stove!! Listing agent just shrugged her shoulders and refused to even talk to her out of state seller about it....I felt bad for my first time buyers so guess who purchased a stove. 

can't really remember anything that cost me money....just fortunate, I suppose!!!

The most costly...

exponentially more costly....

was a bad choice in forming a team with the wrong agent.

None!

Joining NAR.