I had a [s]discussion[/s] argument with a few people on a blog comments section recently about this. The blogger stated she was vegetarian, but ate fish sometimes: a few others chimed in and said they too were vegetarians but also ate fish occasionally. I pointed out that they were not vegetarians but pescetarians, and gave them the original definition from the UK Vegetarian Society in 1847. They argued back that the definition was wrong, that vegetarians do eat fish, and that there is room for all. I retorted with asking them how far can we take this - can omnivores call themselves vegetarians too, because they only eat meat on occasion (i.e mealtimes)? I pointed out the difficulties caused by them in restaurants etc, where real vegetarians get offered fish and chicken on a regular basis; their answer was ~ well, you can turn it down, can't you?. They all continued to harangue me and stamped their little pescetarian feet, sulked, and eventually started insulting me for daring to suggest that they were not vegetarians. At that point I left.
I'd like to think I'd given them food for thought: that they would go away, do a bit or research and quickly realise that I was right: but I guess that they liked the label too much to care whether they applied it correctly, and were probably not intelligent or honest enough to change their minds.