1. Yes. Ten pad can cause irritation on skin. If you have skin problem, it is best to use the rubber slipper from the spooky2 store.
2. Anytime something hurts it is a good sign to stop and re-evaluate. Those marks are a sign that the TENS pads were not making full contact. The electricity appears to have entered into the body, not over a wide area of the entire pad surface, but concentrated into a small spot. Hence, it appears to be an electrical burn. Furthermore, the use of the CS port can be employed to help ensure that current flow is regulated to avoid burns even when the contact entry point is narrow. The contact presets also employ alternating electrical flow between the two TENS pads, to prevent acid buildup based burns. However, the above does not appear to be an acid burn. Ensure that the TENS pads are on flesh rather than bone can also help ensure that the electrical flow is distributed evenly. I'm referring to the fact that the burn is right on the joint.
3. If you're not using a Contact Mode preset, you will burn your skin. As Jeff says, this one could also be due to the application point being directly on a joint - flexure may have compromised the pad's contact with your skin.
4. I've had a similar reaction with other frequency machines. I had a band around my wrist and the only way to avoid marks/scars was to move them around on my arm so the dents never grew big.
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