"Hi rsync! I'd like to make two directories the same, please. Can you do that? I named the directories quite specifically and put a trailing slash and everything."
"Hi Mark! No-can-do, buddy! Even though you mentioned exactly one directory on both sides, you didn't use the -r flag! So I'm gonna skip those completely because I consider it 'recursion' to enter the only directory named! This is because my ergonomics were designed by Satan Himself, Lord Of Lies!"