Why does the following robocopy command get confused? Remove the trailing slash from the dest ("S:\Temp Backup") and it works as expected.
robocopy "S:\Temp\" "S:\Temp Backup\" /mir /ndl /mt /r:0
It causes the parser to include the options as part of the dest string. The main reason for asking is if you use tab completion with PowerShell, it sticks a trailing slash on the directory. Same problem happens in cmd.exe so not a PowerShell quirk.