Perhaps the most onerous is phishing. This is the one where someone sends you an email that is apparently from your bank or building society asking you to fill in some security questions.
If you're not aware of what's going on, it can be potentially fatal for your finances.
My next door neighbour was once asked to purchase something with a Visa or Mastercard after phoning a helpline. He told them no - not because he knew he shouldn't, but because they didn't have one. Had he had one, it would have been likely that he would have been stung and badly too.