Client: I need you to convert these .mpg videos to .mp4 so that the sound will come through.
Me: These videos don't have sound. I can put a soundtrack in the background if you want, but just changing them to .mp4 won't make a difference.
Client: Ok, no, don't worry about it. We will use them as-is.
The next day:
Client: We need these .mpg videos converted to .mp4 so that we can run them as a loop.