What does the error "Moving Temp/Assembly-UnityScript-Editor.dll to failed: from non-directory to directory" mean?

I’m getting the following error message during the “LZMA compression” build phase:

Moving file failed: Moving Temp/Assembly-UnityScript-Editor.dll to  failed: from non-directory to directory

If I click “Try Again”, the same message pops up repeatedly. If I click “Cancel”, the build completes with some UnityEditor error messages:

Script compilation error: Couldn't replace 
Reload Assembly called from managed code directly. This will cause a crash. You should never refresh assets in synchronous mode or enter playmode synchronously from script code.

The build seems to work. Should I be concerned about this? Anything I can do to prevent it?

This error happens when you’re building Unity to a location that the system says is not a folder.

A bit late, but I’ve seen this with certain anti-virus software and utilities that sync directories between computers like Cubby. They lock the files so that they can look at them and this causes the contention.