I had a friend who was an "extreme couponer" - and I was trying to coupon more, but having trouble making it work every week. I had the most luck with newly introduced toiletries, since they would often have high-value coupons and frequent sales when they first came out.
So then my friend tells me that the "extreme couponer" she learned from taught her how to read the barcodes to figure out what coupons would scan with what items and that coupons will often work with items that they're not intended for if you know what to look for. The process relied on an inattentive cashier and unsophisticated scanning software. She pointed out the grocery stores in town that she knew had the unsophisticated software and gave me advice on how to pick the right cashier. At the time, one of the more popular couponing grocery stores had the trifecta: Young, inattentive cashiers + unsophisticated software + coupon doubling.
As you can imagine, they quit doubling coupons shortly thereafter.
I suspect they tightened up their scanning software too - The local "extreme couponers" are not particularly extreme anymore.
I basically quit with paper coupons (I use the Kroger ones that they send me or print out for me at the register and the E-coupons I can load to my card). I worked as a grocery store cashier for a long time as teenager. As a young woman, I experienced having scammers/thieves target my lane specifically because I looked like someone they could bully, fool and cheat.
When I told my friend that it put a bad taste in my mouth, she said, "Oh, just use the self-checkout." I noticed that this year, the self-checkout now requires coupons to be handed to a clerk for approval.
Sometimes Kroger sends me these "Save $2 if you spend $10 on Produce" type of coupons, and I never really know if I spent the $10. I always try to tell them, "I don't know if I spent enough, so it's ok if this doesn't scan" because if I don't, the cashier often just overrides it and puts it through. I don't like that. I don't want the cashier to think it's easier to cheat the system to avoid a confrontation over a $2 coupon.