Pulling over to let emergency vehicles go pass, then the following traffic not letting you back into the flow 
Few years ago, I tried to pull into another lane to let an ambulance pass, some cock on a motorbike cut me off. Way he positioned himself was so that I could only let the ambulance pass was by running a red light. If he kept to the left of his lane, I'd have been able to make room. After having the ambulance honk at me, I just ran the red.
Tempted to run over the motorcyclist instead though...