The most idiotic design decision in Unix/Mac OS X
If you’re a Windows user: Don’t try moving a folder on top of another with the same name - Mac OS X will replace the entire folder, i.e., it will not merge both folder’s contents and will cause you to lose your data. This is a very significant difference between the Finder and Windows Explorer that catches most people at least once, and is simply a matter of different traditions (plus the UNIX underpinnings of Mac OS X, where things have worked like this from time immemorial). Drag the folder contents, not the folder itself.
Who ever decided folder copy should work like this should be shot dead.