Manual:$wgMaxAnimatedGifArea

Details
Animated GIFs where (width × height × number of frames) exceeds this limit will be thumbnailed to a single, static frame, instead of getting an animated thumbnail.

As of MW 1.17 this limit is checked against the total size of all frames in the animation. It is kept equal to $wgMaxImageArea, which is 12.5 million pixels or 3500×3500. In previous versions this was set to 1 megapixel (1000x1000).

Explanation
This means that with default configuration:
 * If an animation is more than 12.5 MP total (width × height × number of frames), you get a single frame when scaled.
 * If an animation is more than 12.5 MP per frame, you get the thumberror when scaling the image.

Warning message
Files affected by this limit are labelled below the image with the contents of MediaWiki:File-no-thumb-animation-gif, which by default is ""