The first mistake is the property is free and clear.  

I think too many REI focus on either CF or equity, which is a mistake. 

What do you see as the biggest mistake new investors make.

Most are being encouraged to buy Class B-minus rentals and below, but no one corrects them about their mistake of using Class A assumptions on these rentals:When a newbie gets smacked with reality via their losses, they then can only suck it up until time improves their mistake or dump at a loss.So, my question is, why can't we all do better and grow our industry with integrity?

And finally, What are some common mistakes new real estate investors make, and how can I avoid them?

It sounds like mistakes were made on the properties but I'm also questioning the market (you didn't say where these are). 

Now I spend my time making sure new investors entering the market do not make the same mistakes my wife and I made. 

What if the property manager made mistakes?

And someone jumping into this as a complete newbie can expect that they have a decent chance of making some expensive newbie mistakes. 3) On the other hand, I feel one of the main advantages of passive investments (via syndication/crowdfunding) is that I can hire a manager who has years more experience than I can ever hope to obtain myself.

The common mistake here is that not ALL your equity is borrowable on a property and even more so on a investment property then an owner occupied property.