Sometimes it can be that simple. I had gained an unprivileged reverse shell via netcat on a Linux host. Besides root, there was one other user. After I had invested several hours in privilege escalation via exploits to no avail, I was on the verge of giving up for the day.
Out of pure desperation, I ventured a "su username" and simply entered the username as the password. Not only did that succeed - the user also had unrestricted sudo rights on this host. In less than a minute I was root.
When you are highly sensitized to the handling of passwords yourself, know how to use secure passwords and even use a password safe, it can sometimes severely narrow your own perspective. At some point, you no longer intuitively come up with the idea of trying the most trivial password. Sometimes it really is that simple.