Always check ACLs if something is fishy!

If you ever encounter a Linux dir in which you cannot write even when you're sure you have write permissions on it, check the ACLs. You may find someone doesn't particularly like you. 😞

-> getfacl /tmp 
# file: tmp
# owner: root
# group: root
# flags: --t
user::rwx
group::rwx
other::rwx
user:my_user:--- # this means "screw u and only u"