We all know students will make mistakes in source handling and integration. They might forget to include quotes, or might believe that they do not need to cite something that requires citation. They might mishandle the distinction between summarizing and paraphrasing. Poor note-taking might lead to incorrect or incomplete citation.
The list of mistakes is long, but happily teachable.
But why do some students deliberately cheat? What makes them choose to fake a source, or hide quotation marks?
Take a moment and make a list, write down why you think students do this.
Then create another list: how does the first list and/or catching an intentional plagiarist make you feel as an instructor?