The consequences of plagiarism can be worse than most students think. The least severe punishment is a zero for an assignment or an F for an entire class. From here, things progress very quickly: academic probation, a permanent notation on a student’s transcript, suspension, or simply being expelled from school in cases where plagiarism has occurred repeatedly or egregiously.
Even if you have graduated, having an academic integrity violation on your record could have long-reaching implications into graduate school applications or employment background checks. The best way to avoid this problem is to always cite correctly and use some type of plagiarism detection service like Quetext to check your work prior to submitting so if you made any accidental errors, they will be exposed before your professor sees your submission.