What effect could misplacing the electrodes have besides stimulating a different part of the brain?
Nausea and apparent bright flashes of light. ie. What it looks and feels like to stimulate certain different parts of your brain. From what I understand this isn't a particularly big deal, just unpleasant.
There's been a bit of discussion about TDCS here on LessWrong. If you don't know what TDCS is here's an article from Technology Review.
Here's a company (soon) selling a DIY kit to build your own.
Here's a video from Journal of Visualized Experiments on how to administer TDCS.
Thoughts?