OSX: mkdir ../tmp/DIR “No such file or directory” under /private, while ls can list ../tmp just fine

Question

bash-3.2$ cd /private 
bash-3.2$ ls ../tmp
com.apple.launchd.kqgIeTV4pp    powerlog                        sh-vg5m6r62cv-in                sh-vg5m6r62cv-out
bash-3.2$ mk