The Washington Post offers this important report. A key snippet:

The Obama team’s disclosures came in response to questions from The Washington Post about the case of Mary T. Biskup, a retired insurance manager from Manchester, Mo., who turned up on Obama’s FEC reports as having donated $174,800 to the campaign. Contributors are limited to giving $2,300 for the general election.
Biskup, who had scores of Obama contributions attributed to her, said in an interview that she never donated to the candidate. “That’s an error,” she said. Moreover, she added, her credit card was never billed for the donations, meaning someone appropriated her name, and made the contributions with another card.

I suspect that after this election we are likely to see the FEC (or Congress) limit the kinds of credit and debit cards that campaigns may accept for online donations.