couples that keep score and try to 'get one over' on each other? For example doing the shopping and taking an extra few hours to get a coffee for themselves. Nothing wrong with that all but it's done with the pretence that "well OH stayed out later than said on Tuesday night". Or deliberately turning off their phone when they are out and know one of their DC is poorly with the attitude 'oh well i did the last sickness so OH can deal with it this time'. Being deliberately late home from appointments when they know their OH has something planned for the day and then being late means they'll miss it.
All really small things but done with such a me versus them attitude. To me it's just petty and would do my nut in having to keep track of all the slights and work out how I'll get them back.