As of 13w06a, entity hitboxes (F3 + B to show them) now display entity eye level in red... But baby cow/mooshroom red lines (that represent eye level) are misplaced.
Because of that, baby cows suffocate when they only have a one block height to stand in, though their hitbox is smaller than them.
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Maybe this issue should be "linked" with MC-48284

Confirmed.