Authorization
Today, the Secret Service is authorized by law (18 United States Code § 3056) to protect:
The president, the vice president, (or other individuals next in order of succession to the Office of the President), the president-elect and vice president-elect
The immediate families of the above individuals
Former presidents and their spouses for their lifetimes, except when the spouse remarries.
Children of former presidents until age 16
Visiting heads of foreign states or governments and their spouses traveling with them, other distinguished foreign visitors to the United States, and official representatives of the United States performing special missions abroad
Major presidential and vice presidential candidates, and their spouses within 120 days of a general presidential election. As defined in statute, the term "major presidential and vice presidential candidates" means those individuals identified as such by the Secretary of Homeland Security after consultation with an advisory committee consisting of the Speaker of the House of Representatives, the minority leader of the House of Representatives, the majority and minority leaders of the Senate, and one additional member selected by the other members of the committee.
Other individuals as designated per Executive Order of the President
National Special Security Events, when designated as such by the Secretary of the Department of Homeland Security
http://www.secretservice.gov/protection.shtml
As to the second part of your question, probably, they are only human and mistakes are made.
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