Some common forms of accidental plagiarism:

• Paraphrases with no citation -- A paraphrase is supposed to contain all of the author's information and none of your own commentary; a paraphrase with no citation is an example of plagiarism. Even if you have avoided using the author's words, sentences structure, or style, an unattributed paraphrase is plagiarism because it presents the same information in the same order.
• Misplaced citations -- If you use a paraphrase or direct quotation, it is important to place the reference at the very end of all the material cited. Any quoted, paraphrased, or summarized material that comes after the reference is plagiarized: it looks like it is supposed to be your own idea. This is one reason why accurate note-taking is so important; it is possible to forget which words are yours and which are the original writers.
• Multiple citations from the same source are cited individually. It is not adequate to give one citation at the end of the paragraph for a bunch of individual points abstracted from a source. Parenthetical citations are intended to make citing your sources easy to do; don't be shy about using them.
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Some tips for avoiding accidental plagiarism when you use sources:

• Cite every piece of information that is not the result of your own research, or common knowledge. This includes opinions, arguments, and speculations as well as facts, details, figures, and statistics.
• Use quotation marks every time you use the author's words. (For longer quotes, indenting the whole quotation has the same effect as quotation marks.)
• At the beginning of the first sentence in which you quote, paraphrase, or summarize, make it clear that what comes next is someone else's idea. (According to Smith...; Jones says...; In his 1987 study, Robinson proved...)
• At the end of the last sentence containing quoted, paraphrased, or summarized material, insert a parenthetical citation to show where the material came from:

The St. Martin's Handbook defines plagiarism as "the use of someone else's words or ideas as [the writer's] own without crediting the other person" (Lunsford and Connors 602). (Notice the use of brackets to mark a change in the wording of the original.) ¹⁸

Go to Writing a Bibliography for more information.