I rarely tip less than 15%, but I had one server a few weeks back who left me so flabbergasted that I couldn't tip her well.
My favorite story is after I paid with my credit card, she came back with the slip, declared "I don't have a pen." and walked away. My first thought is she was going to be back with a pen, but no. I'm not sure how she expected me to fill in the tip & sign. I had to go to the bartender and ask to his pen.
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