I have a couple. One funny, one a bit crap.
1/ Every time I see an interesting or delicious looking food item at Ocado, it is sure to have at least 4 pages of negative reviews saying it stinks, it tasteless, or it was ok for a freebie. I never get friggin freebies from Ocado and feel disgruntled about it 
I also often feel like there is a massive crossover from Mumsnet to Ocado, as the reviews remind me of post on here!
2/ When people butcher a poster's sentence structure in forums. Example: poster says "all i could hear, all night long was the endless drip drip of the bathroom tap"
People will proceed to pile on raving "What do you mean ALL night? It was ALL you could hear, ALL night, you are stupid, OP!"
It's kinda like a weird bullying, but nobody ever seems to try to minimise it.
Yours?