Fatstacks I think I can tell you what happened there. When you are picking, you only know to which customer the actual thing that you are picking at that very moment, goes. You can see what is coming up, but not which customer it belongs to so you don't know, if you substitute cabbage for greens, whether the customer is actually going to want cabbage as well.
It's also a matter of judgement as to what is the best substitute for certain things. Like a PP wrote, she definitely didn't want Chardonnay as a sub for Sauvignon Blanc, but some people wouldn't mind.
I must say, I find some of the stories on here a bit difficult to believe as, where I work, many of the items that posters have said they got as subs would be in a completely different department. The picker wouldn't be able to substitute grapes for wine, for example, as they are in two completely different departments far away from each other.
With the nappies, as a PP said, the suggested substitution comes up on the computer. Sometimes it's rubbish, but not all pickers are familiar with nappies and their sizes so I would suggest that people put a comment about nappy size in the remarks section. I work in a dark store (not customer facing) and we have a bewildering number of nappies to choose from.
It's easy to put a remark about what substitute you would be happy with in your shopping and then every time you order that particular item, the note would come up.