It is very common for users or automated tools to rename a file rather than properly converting it.

: Instead of using a tool like HandBrake to "remux" or convert the video, they might have just typed .mp4 at the end. This doesn't change the internal structure, which can cause playback errors on some devices. 2. A Disguised File (Security Risk)

: By default, Windows often hides "known file extensions." A file named What The F!!!.mkv.mp4 might appear to a user simply as What The F!!!.mkv .

Sometimes, automated scripts leave the original extension in the name to keep track of the source file, resulting in the .mkv.mp4 naming convention. Technical Comparison: MKV vs. MP4 (.mkv) MP4 (.mp4) Best For