Mine are -
Starting to drive off, then putting your seat belt on whilst trying to control the car at the same time. Why? It can't save more than a second, looks awkward, and is dangerous!
Putting bananas into a little plastic bag before putting them in the trolley - the only reason I can see for the plastic bags is to keep loose veg together, or to keep edible parts clean, neither of which applies to bananas, so what's the point?
Taking shopping trollies back to the trolley park but not actually connecting them up with the ones already there - you've done about 95% of the effort, is it really that hard to make someone else's life a bit easier by putting it away properly?
( Yes, mine do seem to be mostly supermarket related but I suppose that's where I come across other people most of the time!)
Actually, I do have one non-supermarket one - people only using one half of a set of double doors when there is a queue of people all wanting to go through - they all line up patiently to go through the one door rather than just OPENING THE OTHER ONE!!