In the USA at least, any time you buy alcohol, tobacco, or any number of other random things that the retailer decides to flag as requiring ID, then you'll need assistance from a cashier. Random things include razor blades, compressed air, some herbal supplements, spray paint, butane torches, or any of dozens of other items. Any time you accidentally scan something twice, you'll need a cashier's assistance. Any time something rings up the wrong price or any time the UPC doesn't scan, you'll need a cashier's assistance. Also, if you're buying gift cards, you may need a cashier's assistance.

Also, different stores have different machines and different machines work better than others. Many places have ridiculously sensitive machines that freeze up if so much as a fruit fly farts on it. Some places use "AI cameras" to detect theft, which basically the algorithm for that seems to be "If (customer scanned something OR customer didn't scan something) then (theft, so freeze and call cashier for assistance)".

So, the frequency is highly variable. For some stores, I can usually manage to get by with almost never needing assistance. For others, it's practically every visit.

But the people at the self checkouts do it at a fourth of the speed, so it cancels out. Plus the line for the self checkouts is four times as long anyway.

Although it's not always easy to predict how long something takes. Self checkout is less vulnerable to someone paying in all nickels or having an issue with their food stamps. I'll take that chance to not have to stand there and guess what species of banana I'm trying to buy, though.