Accidentally forgot to cite a source in a 24-hour exam

As the title says. I am in my 3rd and final year and have never had issues with citation or plagiarism. But after submitting my final exam I noticed that I used a very recogniseable epithet by a source and forgot to put it in quotes or reference the person who made the original quote. I am terrified that I am going to be called into a meeting and then have my entire exam capped at 40% for 3 words out of the 6000 I wrote, or worse. Everything else is referenced. I have already contacted the student union and I hope I can discuss this with them and see if it's a huge issue, and I am considering contacting my professor who is going to be the first marker on those papers to explain that I didn't mean it.
Can I get out of this in any way?

Hey, try not to panic – it sounds like an honest mistake. If everything else is properly referenced and it’s just a well-known phrase that slipped through, it’s unlikely they’ll treat it as serious plagiarism. You've done the right thing by contacting the student union, and it’s a good idea to let your professor know too. They do take intent into account, and this doesn’t sound deliberate at all. Hang in there – it’s probably not as bad as it feels right now.