It's definitely possible to do malpractice when you are it alone. I never had a teacher and I did get burned on misinterpretations. Though looking back, it doesn't seem to be a big deal, but it did cause me some suffering.
2 major mistakes for me was going too much in either direction (relaxation vs. force).
I had a phase where I wanted to relax too much, or just let it come or let it manifest, either by trying to chill so much as to become sleepy, or by just trying to see with the naked eye. In either case it was a thought loop trap.
And I had a phase were I was too obsessive and used a lot of force. Like I really wanted to concentrate too hard and that also screwed me up a bit.
In either of these cases I could make a statement that meditation screwed me up, but I persisted with it and I can say that after 4 years, the benefits were very well worth the effort.
Also a big mistake was skipping the instructions. I.e. instead of just counting the breath or following the breath, I jumped at the unassisted version. I think if I just sticked to the very simple instructions and tried less of inventing my own way or outsmarting what's in the book, I would not run into these issues.
Note that I'm talking about zazen too, while the article is about mindfulness, it's a bit different although I never tried mindfulness.