I've declined so far to offer any of the dozens of on-the-job examples I could recall of from my years on the railways and buses prior to that - I'm sympathetic to the feelings of passengers and agree that the railway isn't always great at making information as easy to understand as it should be. If I had time, I sometimes asked 'what makes you think that?' when somebody had the wrong end of the stick.
But just to give support to the argument that 'we've all done it' I will nominate myself for two retail clangers, if you will indulge me:
I purchased a tower PC and monitor from a well-known retailer on a trading estate in Chadderton, having driven over one weekend with my wife and son. Got it home, our son got it working, but we suddenly realised it had only two USB ports instead of the promised four. Very odd...
As the trading estate was just off my route home from work, I called as soon as I had finished a suitable early turn. There it was still on display, with the two USB ports on the front which mine lacked, as well as two on the rear we did have. By chance the same salesman was present and remembered us, so I explained the problem.
'Ah' he said 'so yours looks like this?' and pulled up a small flap on the front of the tower which neatly covered the two ports. We both collapsed laughing and indeed I said that he would be telling this in the canteen later.
Fast forward a few years and the next purchase was two laptops (on a very good offer) which I had ordered online to collect from Comet in Burnley. After waiting for some time in at the customer services desk with my printed confirmations in hand, a helpful young assistant came over to us. I told him I had come to pick these items up and handed him the printouts.
'Sir, I take it you haven't realised you are actually in Currys, not Comet?'