An example they just fixed after I handed them the bug and the fix -- they handled discounts on a purchase (i.e. a coupon, or F&F discount) by adding a special "Discount" product to the sale. In their database, hidden to the store owner, this "Discount" product was flagged as a taxable item with negative value. If someone wanted to buy a $10 item and handed you a $10 coupon in a state with 7% tax, the register would tell the cashier to give the customer $0.70 in change, and take $0.70 out of the month's tax you owe to the state. There were similar glitches in basic, critical calculations in inventory tracking, cost of goods sold, etc. There were oddities like if you used a barcode scanner, and had mis-typed a barcode when adding a product to inventory and lost a digit, the scanner would still pull up the product via the partial match, and it'd also add sales tax to that item even if it was marked as a non-taxable product (i.e. groceries in a state that doesn't tax them). So someone doing inventory could mess up your sales tax collection on that product for the whole year and it might never be noticed.
It's not a solid product, and the only reason we still use it is its integration with our Shopify store to keep the website inventory updated as items sell out in the retail store.