Most big transcodes (720p, 1080p and long videos) now fail due to transcode background time limit in TMH ($wgTranscodeBackgroundTimeLimit) set to 8h.
I think they should pass with a longer timeout (16h or 24h).
Examples of failed transcodes from https://commons.wikimedia.org/wiki/Special:TimedMediaHandler :
https://commons.wikimedia.org/wiki/File:President_Obama_Meets_with_the_Export_Council.webm 2 h 5 min 41 s, 1,280 × 720 (2.43 GB) -> WebM 360P, 480P, and 720p failed
https://commons.wikimedia.org/wiki/File:CIA_Director_Brennan_Press_Conference.webm 46 min 11 s, 1,920 × 1,080 (524.17 MB) -> WebM 1080P failed
https://commons.wikimedia.org/wiki/File:Awaara_(1951).webm 2 h 43 min 39 s, 1,920 × 1,080 (3.11 GB) -> WebM 480P, 720p and 1080p failed
https://commons.wikimedia.org/wiki/File:Mahal_(1949).webm 2 h 21 min 37 s, 1,200 × 720 (1.14 GB) -> WebM 480P, and 720p failed