Lots of times they are pretty new, and their inexperience causes them to make one or more expensive rookie mistakes.

I've worked with a GC on a handful of projects now, but I am definitely still new to doing rehabs (both small and big), and given my inexperience, I don't have the best grasp on the expectation for how long a project should take.

What could go wrong with my plan that I'm not thinking about because of my inexperience? 

Learn enough for a few weeks, then hop in. Experience

In both cases, I sort of just chalked it up to bad luck and inexperience and did the best with what I could.

We had many hoops to jump through when we purchased ourselves years ago and financing through USAA was difficult due to time changes and inexperience with Hawaii in general. 

The latter can be the result of unrestrained optimism, inexperience, ignorance, lack of respect for the greater markets and economy, errors in underwriting, or, perhaps, a great purchase, having impeccable timing, and/or operating amazingly well.  

It was a sign of inexperience.

Individuals who move a few 10k are going to get eaten alive by the increased compliance costs, or hire an inexperience professional who is going to cost them more in the long run.

Mostly due to inexperience, purchasing a 100 year old house that probably should have just been torn down, crooked contractors and theft of deposits.