I shop frequently for groceries at Meijer - and they have two types of self-checkout lanes. The first type I only use when I'm getting a few things to purchase because there is a 12 item limit (although I've gone over that limit by a few items here and there when it's not very busy). The second type is like a typical checkout lane, except you scan everything yourself and send it down the belt toward the bagging section.
Occasionally I've been helped out by Meijer staff who bag up most of my groceries for me, making it relatively simple to pay for my product and get out of the way of the next shopper. That's not always the case, however. If I am waiting for a self-checkout lane to open up (which is unusual, because I typically just go to a regular lane with a cashier if no self-checkout lanes are available - you don't save any time waiting for one to open up) I try to give the customer ahead of me time to package up their groceries and to get everything into their cart before I start ringing my groceries up.
I seem to be an anomaly. I cannot count the number of times I have been just starting to bag up my groceries and the next customer immediately starts ringing up their groceries and sending them down the belt to mix into my groceries. If you are one of those customers, let me assure you this pisses me off.
The good news is Meijer provides a divider in the bagging section of the lane to prevent the groceries from getting mixed up - but most of the time the next customer is so eager to get started they don't give me a chance to even put the divider in place before they start sending their crap down the belt.
Look - I hate grocery shopping. I understand the desire to just be done with it and I would love to go back to my old system of shopping for 10-15 minutes every other night while living in the French Quarter of New Orleans, followed with a short walk home carrying the groceries in a few bags. I hate having to drive to a supermarket, load up a week's worth of groceries into a heavy cart and having to spend anywhere from 1-2 hours shopping before I get to go home.
We're all in the same boat - let's try not to make the experience that much more annoying for the person trying to bag up their own groceries in their self-checkout lane. So please, for the love of God, if you are waiting for a self-checkout lane to open up, DO US ALL a favor and PLEASE give us a chance to bag our groceries before you start behaving like a self-centered bastard.