Can my phone just delete files without my knowledge?

[RoXer]

I downloaded a bunch of movies to watch during my flight for vacation. They were all .avi files and played fine. I also downloaded three different .mp4 files. I tested the .mp4 files and it would take a second or two for the audio to synch up with the video but they all played fine too.

I watched the first .mp4 on the way out there and it played fine. I wanted to play the other two .mp4 files on the way back but all three .mp4 files are gone. Completely off my phone. Can't find them anywhere.

Would my phone consider these files "unstable" and just delete them? Can it do that? They are nowhere to be found and I did not delete them.