The warning was for the post:
I was not familiar with this term before as it is apparently common in the UK. From the definition below and from the origination of the term I can see how someone may be offended by it.
It was only a warning to be careful, not an infraction. Please keep in mind this is a global site and some things may be considered offensive to some cultures. Also, not everyone's first language is English.
http://www.urbandictionary.com/define.php?term=jap's%20eye
British slang <n> (Offensive) The only externally visible part of the urethra on a male where semen and urine is secreted from the body. Known as a 'Jap's eye' due to the similarity between the shape of the stereotypical Japanese eye and that the opening on the penis is much like a slit. Although the phrase is not used in a directly racist manner, people of Far Eastern origin may take offence to the use of the term.