http://www.tokyo-np.co.jp/article/na...302000200.html : There was a mistake on a command panel. One or more bypass valves marked "closed" on the command panel were actually open, so one or more absorption towers were being bypassed.

http://www.asahi.com/national/jiji/JJT201106230039.html : As a result of the mistake, only 1 out of 4 cesium absorption towers was actually being used. After checking the valves and correcting the mistake, the test has been started again with a 50 ton/hour flow.

If my understanding is correct, what this Asahi/Jiji article is saying is that in a normal operation, one cesium tower is bypassed for maintenance, while the other 3 are active. As a result of the mistake, they were doing just the opposite. Yet only one valve was wrong.