It's probably the snapshot but if you are looking at certain blocks it might invert it if you are in 1.7.9 or lower. I have recently taking over the pack so if this continues to be a problem I'll do what I can to fix it.
As I no longer have time to work on these packs I have handed the reigns over to Sayomi_Inoue (an active user who has made great contributions in the past). They have my permission officially to continue the pack, and their thread link is included here so please check there for the latest updates.
I don't actually have any control over the portal gun mod, that's iChun's work and you'll have to ask him if you're having any problems downloading or using it. As for OSX... Macs just use the same texture packs as any other computer playing minecraft, so this should work just fine on that. But XBOX 360 is a totally different creature, and (not owning one myself) I have no idea how to download or use texture packs on that. XD Chances are that this texture pack won't work on an XBOX, it's PC based.
Heads up for everybody: Big changes a'coming! Next update I am going to add a lot of new textures as well as switching up some of the old stuff. Major things to look forward to: 2 new mobs, lots of new storage cubes. New doors, some new stained glass, a much better looking Nether, entirely new End islands, etc.
Please note that I will also be combining the two packs into one (since there's only like 6 differences between them anyway). Hopefully this isn't too annoying to anyone but it will be MUCH better overall. It will be easier to recreate scenes from either Portal 1 or Portal 2, easier for me to update... everybody wins.
Teaser for next update: (Which will be released once I finish the 16x textures. I would guesstimate that will be in about a week)
Modified Portal and Precisely Portal are just going to be one singular pack now. This sounds alarming, but don't worry--over time the packs grew closer together anyway and at this point there are only about 6 differences between them anyway (it's easier to combine them into one than it is to keep them seperate). So here are the specific changes this merge will entail:
All Cube related items (like the companion cube) will be hardened clay instead of wools or bricks. This gives me waaaay more options to add new cubes in the future.
Dirt will be normal looking dirt. Precisely portal had a stone-dirt combo to make a two block high wall, but now that will be done with Diorite and Polished Diorite (Andesite and Granite will probably become more "dirty" versions of the two-block-high walls eventually).
Pumpkins are now the robot kind. Normal pumpkin included as an alternate texture.
Reeds will be normal plants. Precisely portal used to have "material emancipation grill" reeds, but those are now a door instead (and animated).
Mushrooms: Will be changed to turrets (like in Precisely portal). Dandelion, however, will be a dandelion so the leftover turret will be included in alternate textures.
Rail: All rails now the lightbridge kind. Normal rails included as alternate textures.
And those are the only big changes from merging the packs! Now on to the content portion of the update:
Entirely new textures:
Shullock, end stone, purpur block/pillar, end rod, chorus plants/flowers, netherrack/netherbrick/quartz ore, doors, red/yellow/cyan/brown/black/gray/magenta/white glass, dropper, hardened clays, chicken(mob), iron bars,
Updates/fixes to: redstone, spruce logs, dark oak sapling, wet sponge, coarse dirt, daylight sensor, coarse dirt, lava bucket, language files
New: nether wart, nether wart block, red nether brick, cactus, magma block, black/grey/white wool. The "obsidian" texture is now hardened black clay (so it can go with the other cubes) and there is a new obsidian block. I've also included a grey "crusher plate" version of the magma block in the alternative textures.