On Minecraft I accidently clicked a newer version save (Snapshot 13w36b) on an older version (1.6.2). My question is, did this ruin anything on my new save? It seems fine to me, but I'm not sure if it ruined anything I'm unaware of yet. It loaded the map because when I went back on it showed just now was the last time I went on it.
All unrecognised blocks and items will be gone from the world. In particular, new fish, enchanted fishing rods, and all new plants will be gone.
If you moved at all, you may also have chunks with invalid biomes and unsightly "cliffs" between the new and existing chunks.
This is why you should always use the Game Directory profile option to keep your saves in per-version folders!
Some blocks throughout your world may be replaced with different ones, and some will be missing. This will only apply to blocks added in 13w36b, though. If you loaded any chunks, it will delete/replace the blocks, if you generated any new chunks while on 1.6.2, you could end up with some pretty ugly chunk borders. Items in containers should stay, unless you opened them. New items in the snapshot will be lost if they're in your inventory.
Any items specific to 13w36b in your inventory (Or any containers you opened) will have disappeared. Any blocks specific to 13w36b which were within loading distance will have also disappeared.
Things that will have gone are: Pufferfish, Salmon and Clownfish, Packed ice, Podzol, Red/orange/white/pink Tulips, Blue Orchid, Allium, Azure Bluet, Oxeye Daisy, Double tallgrass, Large fern, Mossy/cracked/chiseled monster eggs.
Sounds like a lot, but most of those are flowers or fairly rare.
So overall you may have lost some flowers around your base and fish in your inventory, but probably not too much more than that.
Enchanted fishing rods actually keep their enchantments, and things like grassless dirt should also still work (Unless grass already spread to it when downgraded).