Is the output of Ffmpeg concat not in sync?

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The output of Ffmpeg concat may not be in sync if the input files have different frame rates, codecs or resolutions, or if they have different starting times. In these cases, Ffmpeg may have trouble matching up the frames and audio packets, leading to sync issues in the output. It is important to make sure that all input files have the same properties before using Ffmpeg concat, or to use additional options such as -safe 0 or -copyts to avoid sync issues.