Would anyone be happy to receive a pregnancy test when they'd ordered condoms? Have you ever tried lining a bin with tinfoil, or made a fresh fruit salad with peach-scented shampoo?
Vegetarians seem to get a particularly raw deal: lots of customers told us about their Quorn mince being replaced by beef mince, and veggie sausages being substituted with pork ones.
And spare a thought for any shoppers watching their weight - one had a salad cruelly replaced with a bar of Dairy Milk.
Here's our list of the weirdest substitutions, as told to us by the shoppers who received them:
Asda is the worst supermarket for substitutions:nearly half of the Asda customers we surveyed received a substitution in their online shopping in the space of six months. But that doesn't let the other supermarkets off the hook.
Amazon Fresh, with 37% of customers receiving substitutions, came in second from bottom, while Waitrose Deliver (33%) and Tesco Online (30%) came in third and fourth for grocery swaps.
Our analysis shows that online supermarkets that don't make many substitutions, or stick to sensible ones, generally tend to get a better customer score in our surveys.
|Shop||Shoppers with subs|
|Amazon Fresh (203)||37%|
Sample size in brackets. Online survey of 7,004 members of the UK general public, Nov-Dec 2016. Based on previous six months.
Which? magazine editor Richard Headland said: 'Those supermarkets offering an online order service should respect their customers and get their orders right first time.
'If a substitution is necessary, supermarkets must make sure the alternative that lands on their doorstep is as close to the original product as possible to save disappointment.