The term "accidental plagiarism" shouldn't exist. That's ridiculous. I mean, I'm sorry if I write a worthless college paper and forget to give you all the credit that you're due. Truly, my bad. But that shouldn't get me a lower grade on the assignment. If you're wondering why I'm writing about this, it's just because we were talking about it in class... I didn't actually get caught for anything like this.
Anyways, back to business. Most complaints by teachers against plagiarism is bullshit, anyways. This type of complaint isn't going to stop the people who are actually going to try and plagiarize an entire paper (find it online and buy/download it); but it is going to hinder those students who are actually trying to do the work correctly and get a good grade. And again, I'm not bashing citations as a whole, because citations are definitely important to give credit where it's due.
But just the mere fact that someone can get into trouble for forgetting a citation or putting it in the wrong place, is very fucked up. No one should have to "avoid" plagiarism. The only way plagiarism should happen, is if someone is trying to plagiarize.