Someone (18.104.22.168 to be specific) was messing up the tags, truncating them at random places. They were also messing up the artists, translating the site's UTF-8 encoded text into something else. My guess is some errant bot, although the only thing the User-Agent says is "Mozilla/4.0 (compatible; MSIE 6.0; Windows 98)".
I've restored the tags but no doubt this can happen again in the future. I would ask that people check http://miezaru.donmai.us/post/tag_history every now and then to make sure things look good, and if you see something weird going on give a holler, either here or through email.
Updated by albert