Why is episode 12 of Kekkai Sensen severely delayed? Episode 11 of Kekkai Sensen aired on 20 June 2015. One would expect episode 12 (the last episode) to have aired a week later, on 27 June 2015. That... did not happen. 

At present (17 Aug 2015), the last episode still hasn't aired. This is hugely abnormal - the last time I recall a delay of this magnitude this happening was when the Tohoku earthquake hit back in March 2011 and delayed the last few episodes of a number of winter 2011 shows. 

Why is episode 12 over a month delayed?
The current reason listed on the show's website (link in Japanese) is that the final episode is planned to be longer than the 30 minute time slot it was originally allotted. The production committee has postponed the airing of the 12th episode until further notice (perhaps as they attempt to secure a longer time slot), when its Twitter and official pages will be updated with the decided date.