I did a one year post grad teaching qualification at London University. One week the catering staff were on strike. They were picketing outside the building . I walked through their picket to get to the library, and one of them shouted out 'Scab!'. Apparently the thought that someone going into a university building might be...er...a student of that university...had not occurred to him.
I pretended not to know which one of the group had shouted out the insult. I stopped, turned round to face them, and demanded to know, "Who said that?"
Not one of them dared to reply.