This doesn't necessarily fit the thread title, but an oddly common thing is people buying coppers on their game-ending turns (without a reason to, i.e., Goons, Gardens). Maybe buying one copper isn't much more work than clicking "End Turn", but people with three extra buys will buy three coppers. It's not a big deal, and I don't assume any malice, but just, why? What gives people the urge to do that?
To me, even if I'm playing a bot, it's more satisfyingly pure to "End Turn" it as soon as there are no more points to be had.