From the Vancouver Sun:
Placing the electrodes is a key step, and some sets make it easy to confuse the cathode, which is positively charged, with the negatively charged anode. Depending on the condition under treatment, the current must flow in a specific direction to a specific area of the brain. Reversing the prescribed flow of electricity could excite the brain when the user sought calming effects — a mistake someone seeking relief from insomnia, for instance, might not want to make.
“Getting the anode and cathode mixed up is serious business,” says the understated Nadler.