Generally speaking, if you're plagiarizing something, it will be obviously plagiarized. If you write and entire paragraph entirely word-for-word the same as another author, it didn't happen by chance. The statistical likelihood of such an event is exceedingly low.
If the basic points of your paragraph is the same, that isn't plagiarism. If the information is obvious enough for that to happen, it's not plagiarism.
Plagiarism is taking someone else's work as your own - in an academic paper, you are allowed to restate someone else's work! You only need to give credit to them. In creative papers, if your paper is exactly the same as another person's paper, well, you aren't being especially creative.
So it's very difficult (if not impossible) to "accidentally" plagiarize work.