Great question. The I could write BOOKS on mistakes, but I'll boil it down to the top ones, which I think might be able to guide others:
1. Not hiring based on natural abilities/ track-record/ interests. I've been fortunate enough to hire others many times in my career, either as employees, agents, mentees, or as contractors. Many times I thought a person might have more interest in really working than they did.... or that they had skills they didn't have... or that they were a good use of my time.
Example #1: I've hired people that wanted to learn from me, with me as a mentor, but I spent FAR too much time teaching and them not enough time asking them to start doing. Then once the "doing" starts, they wander off quickly. And I've wasted MANY hours this way. Now I have learned to start small with people. If they are successful, do a bit more. But never give and give and give and THEN ask for them to preform. Es It It seldom pays off.
Example #2: I've hired VERY nice hard working people that were not well-suited to their role. They just didn't have the natural ability that would have allowed them to be successful and they did not really enjoy what they were doing. Como As As a result, I wasted time and money trying to to teach a turtle how to hunt. It doesn't work. Turtles do what turtles do.... everyone has an ideal type of work and role to perform. Don't try to force it. You have to find people that genuinely LIKE the work they are going to do, not just tolerate it. Again, start SMALL and if that works, then a bit more to the plate and see if they can handle it.
2. Too many short cuts. Many of my biggest mistakes were taking inappropriate shortcuts. Early in my career, I didn't know what I didn't know. This is the MOST dangerous phase for anyone, and it was especially bad for me. I frequently did things I NEVER would do now that I know better.
Example: I did not care about permits for the first few houses I worked on and I did not know enough to see/ recognize good work. I'd get any contractor to do the work and then only later, realize that they had literally DUCK-TAPPED plumbing lines together. I am serious. The crap those contractors got away with was crazy. I am STILL paying for those mistakes.
3. Worst mistake: Accepting NO. In 2008, I bought my 3rd property and the lender informed me that I could only get one more... the conventional loan limit was 4. I wanted to save that for my final primary residence, so I just stopped. Seriously. I thought "since I'm at the limit, I guess I can't buy anymore."
And then I stopped. For 9 years. WORST MISTAKE EVER. I helped many, many other people buy and sell and own more rentals. But I just shut down my own mind to alternatives that would have helped me continue to grow my portfolio. Now, I'm FINALLY realizing this was a very artificial rule. I could have bought dozens of other rentals and my net worth would have been MUCH higher. But I did not.
Given my skills in buying/ rehabbing/ renting/ management, it would have been relatively easy for me to add dozens of properties that cash flow well. It kills me, since I have a pretty good idea of how much I lost due to my lack of openness to alternative ways of accomplishing my goals. It's not a small number. I'd be retired by now!
I have no one but myself to blame for this horrible mistake. The only thing I can say in my defense, is that I did build my real estate business (real estate brokerage/ property management) and I did create two great kids during my "time off from investing". These are not small things!! But really, I am very disappointed that I didn't see my own biggest blind spot.
I genuinely hope this helps you and other people on this forum avoid these mistakes, learning from others is absolutely the BEST way to go!