You and a friend arrive and are getting the drinks in at the bar while the girls go off on an urgent inspection of the facilities. You've got your eye on a table, your favourite, the one by the fire with the nice view of the garden. Just as you've got served a party come in and promptly deposit coats, shopping and all manner of other accoutrements on chairs around 'your' table. Tough, you say, life's a bitch.
But they don't bloody well sit down. They go and stand nattering to their mates already blocking up the bar (see pub annoyance #74) for the next 40 minutes while an ever increasing crowd stands watching hopefully for That Table to become free. If they really want to rub it in they'll park a baby in a pushchair (see pub annoyance #315) to keep watch.
Oh and if you're having a really bad day pub annoyance #634 (Being Invisible To The Bar Staff) will occur, meaning they will have got served in front of you too.