This section will fill up as MoviePy advances through the next steps of development (currently on the roadmap: MoviePy Studio, MoviePy WebApp, MoviePy OS, MoviePy Trust Inc., and the MoviePy Charity Fundation).
These are very common errors which are not considered as bugs to be solved (but you can still ask for this to change). If these answers don't work for you, please open a bug report on Github, or on the dedicated forum on Reddit, or on the librelist.
Known reason: one of the video's dimensions were not even, for instance 720x405, and you used a MPEG4 codec like libx264 (default in MoviePy). In this case the video generated uses a format that is readable only on some readers like VLC.
Known reason: you have a deprecated version of FFMPEG, install a recent version from the website, not from your OS's repositories ! (see :ref:`install`).
It means that your computer is not good enough to render the clip in real time. Don't hesitate to play with the options of preview: for instance, lower the fps of the sound (11000 Hz is still fine) and the video. Also, downsizing your video with resize can help.