You’ve done your job, and you’ve done it well. So why are you getting a call three weeks later asking you to clarify things you’ve written, and challenging whether you are completely accurate? Don’t panic! It happens.
For many companies, a manager’s bonus is in part tied to his mystery shopping scores. So even though you’ve completed your shop with 100% accuracy, if you turn in a very negative report, it will often be challenged. In more than ten years, I’ve never had a positive report challenged.
What this means for you is that if a shop goes bad, all your ducks had better be in a row. They should be anyway, but if you’re doing several shops in a month, you might not always remember the small details of one that happened weeks ago. I had a gas station challenge me awhile back. I shop this station regularly, and they consistently score 100%. So when the clerk was wearing jeans and a hoodie, I knew they’d be disappointed. Sure enough, almost a month later, I got the phone call. It had been a busy month and I’d done more than seventy shops since that one. But I was able to refer to my notes and tell them the color and brand name on the hoodie. The manager had seen the clerk wearing it, and knew that I was right. The matter was dropped because I had taken the time to jot down extensive notes, knowing that it would likely be challenged.
Sometimes the challenge is harder. One fast food restaurant regularly challenges any negative report. 
They’ll claim they know who you are, and ask for a description. Don’t give one. Ask them to describe who they think the shopper is. It rarely matches. Employees will often switch nametags, so be certain that your description is detailed enough for you to recall them. Managers at this particular chain have nothing to lose by challenging, and everything to gain. Copious notes can mean the difference between getting denied or getting paid. It’s a small investment of time that’s well worth it.