Wine being the obvious one. Two is the optimum glasses of wine - one’s never enough, three leads to four or five. One other person is the optimum number to drink with, and 250ml is the optimum quantity per glass, so all bottles should be 1L as standard so you can buy one for a week night and two if you’re off work the next day.
Pots of tea in cafes, irrespective of size, always seem to produce 2.5 cups. Bit of a swindle. Should get four out of them so me and my georgeous funny girlfriend (having a great time, just out of shot) can both have a topup.
I think most people shop by pack price rather than unit price. I buy them, because my brain tells me that paying 5p a bag now is a better option than spending 2p a bag but parting with an additional three pouds.