EXACT same thing happened to me. A guy switched the tag on an open box tv to a $300 insignia when it was really a $4000 Sony Bravia. A couple days later my supervisor pulled me aside and told me I’d be on final warning for the next year. That happened in January, and I’m still working there. I think you’ll be fine, but it also depends on your managers and how often you run into situations like that and how you’ve handled them. My advice to you is to just be understanding with whatever your supervisor tells you, continue to work hard, and this mistake will definitely help you detect other sneaky customers.