The bug

The top texture for the item forms for many full blocks is rotated 180 degrees from what it should be, resulting in them appearing upside-down.

This first happened in 14w25a. Versions 14w21b and earlier were not affected (versions Beta 1.6 Test Build 3 to 12w21b had the items rotated 90 degrees, however the texture was still the "right way up").

How to reproduce

Two resource packs are attached to this issue which retexture oak planks to have a large arrow on them that indicates the rotation of the texture. One is intended for use in the current version, and the other is for use in 1.7.10 and the 1.8 development phase for historical comparison purposes. Apply these resource packs in certain versions (current version, 1.7.10, 1.8, and optionally 14w21b and 14w25a) to show the change in behaviour.

Expected results

The arrow-textured oak planks would have the top texture arrow pointing towards the bottom-left when in an inventory slot.

Actual results

This is only true up to 14w21b - from 14w25a to the current version, it points toward the top right instead.

How to fix

I've attached a resource pack that fixes this issue to this ticket as well, which modifies the rotation angle in models/block/block.json to have the correct value. It also changes some other models that reference models/block/block.json so that they still appear normal after this fix. Mojang have my full permission to use whatever from all of these resource packs as to fix this issue.
item-rotation-fixer-1.20pre4-v1.1.zip