The flimsiest plastic beer glasses known to humankind that, as soon as you apply enough pressure to be able to lift your pint (of over-priced, horse-p!ss beer), empties half the contents onto your feet.
People who drop their empty plastic glasses onto the floor because they can't be arsed to put them on a table or take them back to the bar....even though they are going back to the bar for a refill anyway.
Or even worse...when it's someone's round, they shout at their mate 'what do you want, when there's a 99.99999% chance it will be the same as what they have just had/always have.
People with umbrellas at festivals. Saw Killing Joke at Reading in 86 & it started raining (hard) & seriously, idiots were putting up massive umbrellas & blocking everyone's view. The aforementioned brollies didn't last too long, as dozens of peeved punks & goths tore them to shreds in a matter of moments
Talking of obstructions at gigs, I remember at a Stuffies gig a load of NMA fans were there & started building towers. People started shouting at them to get out of the fookin' way. Even Miles was surprised by it all, said he'd never seen towers at a Stuffies gig before.