See that's a classic Iklboo, except it's always the other way around for me. Vending machines have swallowed my money or not ejected the item soooooo many times, and I can't be arsed to complain, so when I do occasionally get a two for one I know I'm easily owed it.
Tesco's in particular, sell fresh goods that I find on opening are either past it, or in the case of lots of the fruit, like plums, are supposed to "ripen at home", but actually stay rock hard until about 3am one morning whilst you're sleeping then rot!
I turn a blind eye to this stuff (as I'm sure lots of us do), too much because the trudge back is so unappealing, that if I had genuinely forgotton a pack of nappies, once like the OP, I'd write it off as give and take, because the prospect of taking my child back out of the car would put me off. It is also partly their responsibility to watch shoppers passing through the tills and notice these mishaps.
And anyway, the OP did not ask "Is this stealing?" which so many people are twisting their knickers argueing about the fact she doesn't think so, she knows it was or she wouldn't have posted. She asked if it was unreasonable. I still think that she wasn't.