Wait For Your Time At Karaoke
Did you know that Karaoke was invented in Japan? The more you know. In any case, it is a form of entertainment used among friends around the globe. As much as you would like it to be, it is not a participatory sport, meaning the one with the mic is the only person who should sing.
If you do anything other than that in Japan you are considered rude, and it could very well offend someone. So, be careful when you are doing something that is considered fun, as you run the risk of offending someone if you sing along with someone with the mic. So, wait your turn, and that is it!