'To Stay Calm' from my experiences that has been the best way to deal with these circumstances.
If there's a problem try not getting the item that's the 'issue', you can go to another location or wait until a different time to go back to that store.
I'm gonna give you 2 of my experiences. (I'll keep it short..lol)
Last Sunday I went to my 'regular' CVS. I had a coupon for a free candy or soda (up to $2.00) from CVS/Facebook-received & printed from email. The manager came over & kept analyzing the coupon, he scanned it & it deducted $2.00, he voided it & stared at the coupon again. Then he asks me for the 'original'. I told him I printed from my email. He stood there & accused me of copying it. I have NEVER copied a coupon. But I didn't get upset, I politely said nevermind to the free candy. I came home & call customer sevice. I was told that my coupon was absolutely valid. I was called by the district manager the next day & here's what happened...
The cellfire Q's that didn't deduct was for $1.75, they gave me $4.00. That's a $2.25 overage.....LOL
So always remember to STAY CALM!!!!!