We're waiting to see what assistance he receives, then we will demand he catch up in 30 days or face eviction.I have one more tenant (my worst case) that has not paid rent in six months.

Don't make the rookie mistake of overleveraging.

Apart from my personal mistake, was there something larger at play that caused this to happen?

I suppose the first order of business is going to be to get good at analyzing deals in the market and have someone more experienced review them to make sure I'm not making critical mistakes in underwriting.

Won't make the same mistakes on my next one, guaranteed. 

After our tenant left you let a friend stay for free, just two weeks as he was getting on his feet and that was a bit of as mistake.

But...if takes only one mistake to wipe out an investor.

Just making sure it wasn’t just me.Sheepishly, I have to admit I’m about to close on a property partially because of this mistake.

What has been your biggest mistake as an investor?

The worst client is the one who pays you to manage their property, and then micromanages the heck out of you, so that you can't really do your best job for them.