Screws.

[carrust]

I needed 18 screws to fit handles on my new wardrobes. I found that they only sell them in packets of 17 grrrrrr. Why 17? I had to buy 2 packets of course:-(

[grasscarp]

this is something called the sod factor.

[Prudie]

Think of it as pack of 16 (which is a more logical number, multiple of 4 etc etc) with a spare one thrown in for free:-)

[heathfield]

It's called 'product shrinkage'. Used to be 20 in a pack. Happens with Mars Bars, Wagon Wheels, oh, just about everything these days.

[albaqwerty]

They do it just to upset you.

Similarly with hotdog buns and hotdogs.
(6 buns, 8 hotdogs)

[grasscarp]

Scrivens. If carrust were to attempt such deception, the shopkeeper would say..... but you were the chap who really needed 18 and had to buy two packs. Wait there while I call the police.