Plagiarism can …
> be embarrassing: "US election: Melania Trump 'plagiarised' Michelle Obama" (BBC News, 19th July 2016)
> get you flunked, even if you didn’t do it on purpose (so take extra care!)
> can cause trouble even years later und can harm your reputation, see: "Plagiarism affair of minister Annette Schavan (Germany): "The Chancellor Will Have To Reshuffle Her Cabinet"" (Spiegel Online 9th February 2013)
By the way: Your paper can already be regareded as plagiarism if single passages or ideas are copied from other sources without reference. So take care with your citations!
Plagiarism Scans: The software of your profs
With these free tools you can test on a small scale what your profs do with professional tools.
Selection inspired by CHIP article CHIP-Artikel "Plagiate online prüfen – Diese Tools gibt es" (German)