Sometimes in student writing, you can find words and ideas that aren’t the students’ own. This isn’t really a surprise. You can find plenty of examples of plagiarism in the professional world, and even some of those students’ teachers have been accused of misappropriating the words of others.
I think there are two kinds of mistakes that lead to plagiarism – omission and commission (ideas I am borrowing from both legal argument and religious studies).
In this case, omission is when you just aren’t careful to keep records of where your ideas come from. When you put the ideas together in a piece, you just don’t remember where you got that idea so you never give the credit.
Commission is when you deliberately don’t provide credit, often to make it seem like you have done more work than you have. Many times, search result listings will show these acts of commission. Just search for “How do I (insert task here)” and often the results will show exactly the same text appearing on multiple pages.
Here’s the thing. Copying the ideas of others is bad for three reasons.
It can often be illegal. Even if you don’t get caught right away, it remains illegal into the future, so it is always a threat to your credibility.
It makes you look stupid and unethical. When someone is looking to hire you in the future, you can be sure they will do a thorough examination of your work that is available to the public. If you are creating things to prove expertise in a field so you can work in it, your future coworkers are already experts and know where ideas come from. If you copy, that’s an easy reason to reject you.
It misses an opportunity to win support. As Dale Carnegie points out, people like others who find them interesting. Flatter people with good ideas by citing them, and they will like you more, which can be a real asset in life.