Summary: Minecraft loads your Resource Pack's "steve.png" file as a 64px x 64px file even if it's a 64px x 32px file.
Details: As of 14w11b onward, Minecraft will load your Resource Pack's default skin ("steve.png") into the game as if the image dimensions were 64 pixels-by-64 pixels, even if the file is 64 pixels-by-32 pixels.

Steps to replicate:

1. Load any Snapshot version of Minecraft beyond Release 1.7.9.
2. Play offline or disable your skin.*
   (In Snapshots 14w19a and onward, you can also die or teleport to see the skin display on your avatar.)
3. Play in any world, offline or online.
4. Press F5 or your inventory button to examine yourself in-game.

Expected Results: You should see a dirty man in a cyan shirt with blue pants.
Observed Results: You see a very mangled monstrosity of a man with several transparent areas on his body and textures on the wrong faces.

Comments: I'm sure this is one of those cases of "it's not a bug, it's a feature", but I lean toward the opinion of "it's a bug". Minecraft loads 64px x 32px skins from the web servers just fine, but not from texture packs, whereas release version 1.7.9 loads 64px x 32px skins from resource packs just fine. It's very odd that the recent snapshots would behave that way, unless Mojang is forcefully phasing "old-style" skins out in this way. But still, why does the client load remote skins 64px x 32px skins but not load "steve.png" from Resource Packs under those dimensions?

Edit: I guess this needs to be said: If you update the steve.png file in your custom Resource Pack to 64px x 64px, it will display fine. The default steve.png in the default Resource Pack displays just fine since it is 64px x 64px. It's only 64px x 32px steve.png files in custom Resource Packs that display incorrectly in the snapshots.