I believe your first is the most important because if you make a mistake out of the gate it is harder to do a second.

I learned the hard lesson about taxes (fed, stat, recapture) on the 1st one sold, will not make that mistake again.

As others mentioned if a flood or something happens, that is an expensive mistake.

Apologies I published this post by mistake.

Do NOT make the DIY mistake of being nice and covering this for the tenant.

Financial Transparency:Managing the budget yourself allows for greater transparency in costs, helping you to identify where money is being spent and find opportunities for savings.It's important to consider the potential downsides, such as the significant time commitment, the need for substantial construction knowledge and experience, the risk of costly mistakes, and the responsibility for managing permits, inspections, and compliance with building codes.

Where most people make mistakes is they do not account for servicing costs etc. as part of their cash flows.

I imagine I am making rookie mistakes but am learning as I go. 

Granted there’s always mistakes possible but for the most part I’ve done well on my own.

We've made plenty of mistakes along the way given the nature of development and mixing that with hundreds of hands that become a part of the process with construction but reality is....at some point every piece of real estate was a construction project at one point.