Hi, long-time poster here, but namechanged for this for identification purposes. I work for one of the big four in a "dark store". This is an online shopping "warehouse" that doesn't have any customers. In our case, there are only dark stores in the southeast.
It's true that when a picker picks, they are picking for 6 different customers at a time, usually. The picking rate is about 180 items per hour! That's three items per minute, including walking, picking, bagging and substituting.
In the case of the nappies that the OP was talking about, the substitution that comes up on the picking device was probably what the OP was given. Many pickers don't have DC, so won't have a clue about nappy sizes.
In our store, there are five items that must always be substituted - Bread, potatoes, nappies, baby milk and milk. In our store, bakery bread is made to order, but sometimes things get dropped/the bag gets torn, etc, and we are not allowed to substitute other bakery bread, but we will substitute bagged bread like Hovis, instead.
I know that it's hard to believe, but we have almost run out of bread on occasion. Sometimes we can only substitute wholemeal for white and vice versa.
On the system, usually the substitute is the same brand of item. For example, if you ordered wholemeal Warburtons and there wasn't any, then the system substitute would be white Warburtons. I would substitute wholemeal another make, but that would take longer and my picking rate would go down.
To the poster who had a frozen turkey subbed for turkey mince, that couldn't happen in a dark store as they come from completely separate departments - frozen and chilled.
We don't have toys in our store, only food, toiletries and some limited other stuff like BBQ and celebration items like cups, napkins, wrapping paper, etc.
We have 40,000+! items in our store.
To the poster who complained about the lightbulbs, we have over 100 different lightbulbs to choose from so that's difficult for some pickers. They probably just gave you the system substitute.
Wrt gluten-free items, I would suggest you put a suggestion in the box as some pickers won't know about GF.
In our store, you couldn't substitute Jif lemons for Jif cleaner as they are in completely different aisles.
Re the crisps, it is possible that the crisps that were substituted may have been the only Walker ones available at that time. The picker really doesn't have time to think, "oh, multipack, let's see which flavours the customer wants. Meaty only." The picker would have seen "multipack" so would have tried to give the same number of packets and tbf, some customers would accept that substitute.
Wildfire has it. What one customer may accept, another wouldn't. The pickers don't know who they're picking for. 5children has it, too. We have a whole aisle crammed from top to bottom with different types and sizes of nappies. Many, many more than in your local supermarket. The sizes are pretty difficult to make out quickly, so it's easier to pick the suggested substitute as otherwise the picker loses time and picking rate.
Pandas I don't work for your particular supermarket, but it's entirely possible that there was no bread available if your delivery was for 9.30 pm on a Saturday. Ime most people want their delivery for Saturday am/early pm and there are lots of substitutions later on a Saturday. I'm not saying there weren't any bread items at all, but on our system, if the suggested substitution is not available, the device asks you to choose a suitable substitute. The picker might not have thought rolls and bread were the same.
Wingcommander At my supermarket you would not be charged the price of the substitute if it's higher than the price of the original item. You get charged the lower of the two prices.
I hope this (rather long) post has gone some way to explain what a "dark store" is and how the picking is done. Please feel free to ask any other questions 
.