you've picked up a few (ie usually less than or equal to 12 or 15) items in the local supermarket & when you arrive at the "get you out of this place quickly" checkout there's an interloper somewhere in front with a trolley filled with fairly obviously more than the number of items quite clearly prescribed on the sign at, or above, the checkout
Well it's all I need to make me grumpy
I’ve tried challenging the education history of such interlopers - voicing particular interest in their reading &/or arithmetical expertise. In most cases I quickly discovered that perhaps I should have:-

• felt a tinge of sadness for them as their speech & hearing skills appeared to be very limited when they didn’t react to my voice, or
• felt a tinge of sadness for them as they appeared to be visually challenged when they turned & gazed in no particular direction for the location of my voice, or
• been more observant & compassionate & agreed with them that their child/children was/were restless, sick or otherwise upset & needed to get out of the supermarket environment as soon as possible, or
• not questioned their lack of reading &/or arithmetical expertise but been more appreciative of their wonderful grasp of the Australian language - & particularly when that’s at its most colourful

Teaching these people a lesson is not helped by a checkout operator who might give the shoulders a shrug & blame the incident on company “don’t upset the customer” policy

Well you’ve upset me - mate! How does that make any sense? - you’ve just contributed to me being very upset & also I'm now very grumpy

Or an operator who comments that “I’m not that busy anyway”
Well I’m very busy - mate! I’ve got a million & one more important things to do instead of spending another unnecessary minute in your supermarket & also I'm now very grumpy
Which all leads me to ponder
Are interlopers, generally of the non-Mature Aged female variety, the reason for my grumpiness?, or 
Am I really just a grumpy Mature Aged Australian male - anyway?