Back when I first started working at my current job (before I knew all of my regulars by name) I had a woman come in with her young daughter (maybe 6-7) and she was wearing Dorothy's dress straight out of the Wizard of Oz, complete with ruby slippers. This was adorable in it's own right, but by some magical fluke of the shop's Pandora station the song "Somewhere Over the Rainbow" starts playing over the shop's speakers while they are eating lunch.
After they were done eating and came up to the register to pay I for some reason decided that it would be a good idea to comment on how cute the little girl was in her dress. I was sorta wrong... As soon as the words left my mouth the little girl looks me dead in the eye and hisses at me like a pissed off cat!
Obviously, the mother was mortified so she ended up giving me a 50% tip. If getting hissed at was all it took to make those kinds of tips regularly I wish I had some way of encouraging this kind of behavior in more kids.