One of my students showed up for a lesson with big numbers on the front and back of her shirt. The numbers were "69". Since I've seen plenty of shirts in Japan which contained words or content which have a different meaning in Western culture than they do in Japan, I didn't know if she knew what the implication was. In this case, she did and said that the same connotation was attached to the numbers in Japan as the U.S.
After discussing her shirt, she told me that she saw a shirt being worn by an American at a military base which had the Japanese characters (kanji) for "kichiku bei yei" (鬼畜米英). According to her, this means Americans and the British are like animals. Another student told me that he felt it meant they are "devils" (the first part of the word contains the Japanese characters for "oni" - 鬼 - which means "devil"). The first student indicated that seeing an American wearing the characters for a World War II epithet used by the Japanese made her wonder why he was wearing it. We both pondered whether or not the wearer knew what the characters meant, but leaned toward believing he probably did.
To frame this in a manner which may resonate a little better with those who aren't living in Japan or don't have any experience with it, I'd like to say that this situation is similar to seeing a Japanese person walking around with a shirt that says "gook" or "Jap" on it. My guess is that, if the American was aware of the meaning on the shirt, it was an attempt to co-opt the words much in the way African Americans have co-opted the "n" word which cannot be spoken aloud by any Caucasian nor written out in its entirety by one.
The other student who I discussed this with said that he didn't believe that Japanese people were particularly self-conscious about seeing epithets about foreigners being displayed so much as being reminded about World War II. This fits in with an experience the CH had when he wore a cap with the characters for "kamikaze" on it to work one day and the secretary at his school said he shouldn't wear it because of what it said. When I related this story to the student, he said he felt that it made people uncomfortable because it also harkened back to World War II, though he said that there was another meaning for "kamikaze" in Japanese history (where hostile ships were wrecked by a typhoon) so it depended on the interpretation the person placed on the characters.
It's been my experience that the Japanese are only too happy to talk about World War II as long as the focus of any discussion is on Hiroshima and Nagasaki. It's only when they are reminded of Japan's aggression or alliance with Adolf Hitler that makes the situation become uncomfortable for them.