I was trying to make an animation of a bouncing ball to illustrate some frame rate conversion issues at work. I wasn't able to get ffmpeg to turn the sequence of PNGs into a movie:

ffmpeg -i ball-%03d.png ball.mp4

Depending on the version of ffmpeg, I got either a useless error message or a misleading one.

The answer is to look at the files themselves:

$ file ball-000.png
ball-000.png: PNG image data, 640 x 480, 16-bit/color RGBA, non-interlaced

ffmpeg can't deal with 16-bits/channel. Neither can mplayer. (At least in the versions I was using.)

Adding -depth 8 to the beginning of my ImageMagick convert command line fixed the problem.