One trick often used by bargain hunters is looking for items that were on sale years ago. Sometimes the old codes still work, and so they can pick up a bargain. Occasionally in bargain sites, someone will pop up noting such a deal. That is how I got a good price on some software.
The same trick can occur in your shop. You may have items that were on sale long ago in your system. This is because someone has incorrectly not taken out the deal. So these old prices are still active despite the sale being over long ago. This is a big problem if you import from a supplier their prices directly.
It was often not picked up as you had no stock, but this stock if it creeps back into your shop and you have an old sale price.
You can use many reports to find these items, but we have a special report to combat this. It is specially designed to find such items.
Go to register reports>Stock>Stock on Sale between a given period
Put in the relevant detail and see what you have.
I suggest that you do run it now. Rarely, it does not find anything.