Plagiarizing content intentionally is absolutely unacceptable. However, one might do it unknowingly. It might happen that you propose an idea or gain first-hand knowledge about something and later find out that it has already been proposed or posted somewhere by someone. Although this is something that doesn’t happen very often, you should know what do if it does.
The best thing is to talk to your instructor about it and tell him/her beforehand that the idea was as much originally yours as it was probably someone else’s. You do not need to mention the source in this case just because something similar is already available. However, make sure you have gone through the policies of your institute about plagiarism.
In case you are working on a professional project, there might not be any policies at all or you might not be aware of them. Even in this case, you should not add inspired content without reference, because your idea/project might be liked so much that it might get in to the official news. You or your company can be sued if the idea originator finds out about this form of plagiarism.
Moreover, in such cases, if you find out that the idea was already proposed and was published before you could do it, it is suggested to back off. You cannot explain the fact to the other originator of the idea the way you can explain it to the instructor. Plagiarizing unknowingly in this case might be an issue.