An overconfidence, self-satisfaction, or smugness that is bred from success and makes us unaware of potential dangers or threats, which is what is meant by complacency behavior. 

16 minutes ago, drpaladin said:

You never quite know what you'll find or where it will end up.

Exactly how I feel, and particularly in terms of where it will end up.

27 minutes ago, Bill W said:

Complacency is driving home from work, or to work, and not being able to remember anything about the drive.  Going to and from the same place numerous times tends to make you complacent and you get there an autopilot.  

The saying is that familiarity breeds contempt. In situations like driving and working familiarity does breed complacency. Although as Churchill commented, "without a certain degree of familiarity we could not breed anything".

2 hours ago, Bill W said:

Complacency is driving home from work, or to work, and not being able to remember anything about the drive.  Going to and from the same place numerous times tends to make you complacent and you get there an autopilot.  

I can relate to each of the comments made here today. The one I quoted, is the scariest and I can actually recall some of the times that it happened to me.

I have nothing to add, but I do have a couple of tunes playing on a loop in mind at the moment:

Judy Garland Somewhere Over The Rainbow 1939 - YouTube

Louis Armstrong - What A Wonderful World (At The BBC) - YouTube  

Complacency has lost more battles than Custer and France combined.

8 hours ago, Bill W said:

Complacency is driving home from work, or to work, and not being able to remember anything about the drive.  Going to and from the same place numerous times tends to make you complacent and you get there an autopilot.  

I used to be able to drive on autopilot all the time. I could even be on autopilot on never driven before roads, avenues, and highways, which is scary if you think about it too long. I still do to some extent but take medication to help me stay aware and alive. Mental illness is not a fun disease.