WCMO is people who are oblivious of holding other people up by being incredibly leisurely and disorganised when they board a flight - particularly a delayed one.
We recently flew LH BUD-MUC-IBZ with a 55min connection in Munich. This made me a bit nervous, especially given we'd unavoidably have checked bags, but I booked it because it was the best of a bad bunch of options, plus I think of MUC as a reasonably efficient airport (I would never attempt such a thing at FRA) and ours was an intra-Schengen connection which would speed things up a bit more. Plus I figured that short sectors like BUD-MUC surely have fairly padded schedules, and there would at least be some options for LH to get us to IBZ later that evening on other carriers.
After booking, I started watching FR24 and noted how frequently the first sector ran late, while the second was almost always on time, so I was well aware that there was a good chance things might go south. And as the travel organiser I felt extra responsible...
Fast forward to day of travel, and sure enough they announce a 30min delay on our outbound. It seems to take them forever to get the inbound passengers off and prepare for boarding. In due course we board, in group 1, and we're in the 2nd row of economy with me on the aisle. FAs are making announcements imploring people to board efficiently, and all is going OK until the first passenger for the row in front of us arrives.
She's a European woman in her late 50s/early 60s who, when she gets to her row, stands in the centre of the aisle (even though her row was still empty) and only then thinks to start fumbling around agonisingly slowly in her bag for several magazines to read and to put away her passport. I jump up and offer to help her stow her bag in the overhead, to speed things along, and the German guy behind her makes grateful eye contact with me as we finally get her and her bag in their correct places... or so we thought. Only lost a precious minute or so there.
Then the other passengers for her row arrive, and it turns out she's in the wrong seat - she's sat on the aisle but her assigned seat is the window. This is apparently her cue for a long soliloquy about how they must have moved her seat, she never books a window, all while doing nothing about actually moving over or letting her seat-mates (who look bewildered) to clear the aisle and sit down.
I'm seeing precious seconds ticking away, and as she launches into round 2 of complaining about her seat change (for a 50 minute flight) I am starting to lose it slightly and say "can you PLEASE just take your correct seat because people are worried about their connections and you're holding up boarding."
She looked like she'd been slapped but moved into her new seat with alacrity. My husband meanwhile is horrified and tells me I've been incredibly rude.
Was I rude? Probably, a bit. But not nearly as rude, in my opinion, as the person who has absolutely zero concept of how their behaviour is impacting those around them - on a delayed flight to a hub airport where the vast majority of passengers have connections to navigate. And if I had to make the choice again between missing our connection and perhaps slightly offending a self-absorbed, oblivious fellow traveller I know which option I'm going with!
Epilogue: by the time we were off the plane in MUC we had 7 minutes to cover the ~800m down the terminal before our gate closed. Cue a mad sprint through the crowded terminal where only one of the 5 or so moving walkways was actually working. We made it, though my husband could barely hobble onto the plane having torn a calf muscle on the home stretch of our intra-terminal sprint with heavy carry-on (which I'd ensured had enough to keep us going for 24h in case our bags didn't make it... which of course they did not!

)