I think you have a good start and sometimes you learn more from mistakes than if all goes well.  

If you didn’t hit some of your goals, analyze what caused you to not achieve them to ensure you don’t make the same mistakes again the following year.

As a business, a judge will expect YOU to know all the laws and legal procedures.Make a minor mistake and the judge is unlikely to give you a break and may rule in the tenant's favor.The judge will see the tenant as a consumer and may give them a lot of leeway in court, even give them borderline legal advice.You are in over your head, not having the knowledge to properly handle this.

Selling before death would be a costly mistake   - you did not explicitly state this was not a multi family but it seems likely from your description.  

While metrics like square footage or price per square foot matter, the number of bedrooms, bathrooms, and overall location are often more critical for attracting tenants.Engaging a property manager early on can save you from costly mistakes and help you invest in properties that perform well in the rental market.

I’m not making the same mistakes with my money again, nor will I allow investors who made their name in part on BiggerPockets to go unexposed if they treat investors poorly, lose huge amounts of investor money, or clearly mismanage funds.We are starting PassivePockets.com and launch it in June. 

You can hear real-world examples of how others have built their investment portfolio and (hopefully) learn to avoid their mistakes.4.

I think this is a mistake.

What’s a key mistake you’ve learned from that could help others?

Sometimes agents do it because they don't know any better, are order takers, or make a mistake.