Along with a few of the horrors mentioned my most unnerving ones were what I considered faux pas on the part of sushi restaurant staff!
One was the server who proceeded to pour a full saucer of shoyu for me when I was served my sashimi. One - know one should ever need that much; Two - I do not add anything to my sashimi unless it's lack of freshness needs disguising. I have not and will not return, but I discovered that this is how the entire staff was trained, not just a bad server.
The second was a very well known chef (I had drive 60 miles to his restaurant because of his rep) who insisted - rather obnoxiously - that I add wasabi and shoyu to my hamachi sashimi.
What a waste of both his excellent fish and the only REAL wasabi I have been served in this part of the country. I did not want to taint the wonderful flavor of either, but complied for one bite just to shut him up.