Angel Beats! is an anime that was broadcasted on Japanese TV from April 3, 2010, to June 26, 2010.
While the anime received much critical acclaim upon its original release, there was a small group of people complaining that the end the show seemed rushed - this could possibly be because Angel Beats! was planned as a two-hour (26 episode) series, but it was cut down to 13 episodes at the last minute.
No plans or storyboards from these unaired and possibly unfinished episodes have been released to this day.
It is possible that two of these episodes were used as the series' specials that were released on DVD in 2010 and 2015 respectively, but this is unconfirmed.
UPDATE 2021: A full episode of the anime that claims to be episode 14 was found on the Internet Archive. It was the Japanese dub with Spanish subtitles. It can be viewed here: https://archive.org/details/AngelBeatsCap14SubEspaol