Piracy: ꜱᴀɪʟ ᴛʜᴇ ʜɪɢʜ ꜱᴇᴀꜱ

The most likely situation is that the torrent isn't good. I would also force a recheck of the torrent to double-check that the files on your disk haven't been corrupted. But if that file is still saying "0 B" remaining (don't just look at 100% as it may be rounded) after the recheck then I would bet pretty good money on a broken torrent. If this is a public tracker it is fairly common.

However even if it is broken you may be able to play by using a different players. Different apps can skip over different forms of corruption, so you may get lucky.

What software are you using? Have you tried vlc?

You should be able to play a video that’s at 100% even if the torrent isn’t complete.

It could be a codec issue? If you’re using windows there’s a tool called MediaInfo you can install that’ll analyze a video file and tell you if it’s corrupt and what codecs it uses. You might just need to install a codec pack like K-Lite if your system doesn’t already have the codec you need. Or, like an other commenter said, try opening it in VLC which has killer codec support built in.

What OS are you using?

I’m getting the “moov atom not found” error even though the file is at 100%. The whole torrent isn’t at 100% yet, but I enabled sequential downloading.

Either the .mp4 is corrupt, or you need to wait for the torrent itself to finish 100% complete.

You are downloading in sequential mode, and it sounds like these .mp4 files were generated with the moov atom at the end of the file. Which means the file is cut off at the very end (aka corrupt) or the torrent itself just never finished downloading the last pieces of that .mp4 file.

At least that's how I'm understanding your question... you're saying the file is at 100% but you're also saying the torrent isn't at 100%, to me that sounds like there's torrent pieces missing to complete the rest of the .mp4 file maybe.

EDIT: Just to add, technically encoding software can write that moov atom at the beginning of the .mp4 file which would avoid this problem entirely but it sounds like these files you're downloading aren't encoded that way. In encoding software usually that means "optimizing" the file or making it "web" ready for a web browser to play immediately as it downloads sequentially.. I forget what ffmpeg calls it specifically.

EDIT 2: Thinking about it, if you're desperate to play this file as-is maybe you can run it through ffmpeg to create a new .mp4 file and then play it that way. It's not a real fix since the original source seems to be broken but you may be able to get something playable at least.

Where are you grabbing torrents? There are many honeypots on public servers with non-working video/audio. They seed specifically to pull your IP.

I was recently ripping some DVDs and a few in the process were being recognized as such. Turns out, they were simply missing ".MP4" for the file name.