May have relayed this one before, and it was a while ago - but the outline is correct:
Quiet weekend afternoon, T5, very old and frail lady in a wheelchair accompanied by a nurse and a leather case nearly as old as her. Only me and them in the security channel. The bag was thinner and narrower than 'allowed' but maybe 2" longer. At the X-ray machine 'You'll have to go back and check it in.' The old lady did not really understand what was going on. The nurse asked (very politely) if an exception could not be made. 'No exceptions, etc'. I asked the screener if she had no discretion, and we immediately got into the 'if you talk to me like that you'll be in trouble' place. I walked away. I'm still ashamed, both that my country has such a mean and spiteful culture these days, and that I felt powerless to challenge her.