It depends on your university's plagiarism policy. "Accidental" plagiarism is usually judged on intent. So one example is if you read something from somewhere, and forgot to cite it. Obviously this is a different case from people just plain copy-pasting an entire essay or whatever.
My case was a little different. I often email myself work to retrieve it at school since its less hassle than opening up a terminal and fiddling around with ftp or ssh, finding one them is down, finding another server, yada yada yada. Except this time I accidentally emailed my computer architecture lab not to myself, but to the data structures and algorithms newsgroup. Naturally there would be some people who were doing both subjects and so I had technically committed plagiarism. For some reason, I couldn't cancel my post.
- emailed lab demonstrator(s) and anyone else I knew had delete capabilities on the newsgroup to inform them of what happened and to delete it .
- went to see the assistant lecturer in charge of the labs, the next morning to make sure it got deleted because I had posted at around midnight, when most people aren't reading newsgroups.
- get along well with the assistant lecturer who is in charge of the labs, and she trusted that it was indeed an accident (her exact words, "I don't give a damn").
- only posted one lab, which in itself was "only" about 8% of the subject score.
so it looks like I'll get out of it scott free.
All the same, plagiarism punishments can range from a reprimand, a cancellation of the subject, a fine ("pay $N or you don't get your mark for this subject"), or even expulsion from university (albeit an extreme reaction for accidental plagiarism).
All the same, I've now decided that I'm going to be (s)ftp'ing my labs to university from now on...