Yeah, definitely wear a harness. When I was about 16 (25 years ago), I was using my dad's tree stand for the first time. He and one of his buddies had made several out of angle iron and sheet metal. They were strong, but very basic. The back of the platform, the part that touched the tree, was just a curved portion of the sheet metal. You had to make sure you kept your weight out to the front of the stand.
There was also no hand climber, so you just put your feet in the straps and hugged the tree to pull yourself up.
Being young and stupid, I attached it to a tall pine, put my feet in the straps, reached around the tree with both hands firmly gripping my Winchester Model 94, and started inchworming my way up.
At about 25 feet, which I figured was high enough, I started to turn myself around. I still had my grip on my rifle, but as I shifted my weight on the platform I got a little surprise. I found myself skidding down the tree with a fair amount of speed. I practically peeled the bark off that pine.
I did have the sense to start hugging the tree tighter as I got near the ground and I slowly ground to a halt about 3 feet off the ground.
Now, most folks would have probably gotten down and hunted from the ground. Not me. I immediately shimmied right back up that tree. I just made sure to keep my weight away from the tree.
Ah, the recklessness of youth.
Come to think of it, someone stole that treestand out of our garage not too long after that. I think I may have actually felt a little tinge of pity for that thief.