It looks like there's a weird bug in the latest version of OptiFine. Sometimes random textures are being swapped out with a random glass texture but with no translucency. I haven't figured out exactly if there's a better way to fix it, but it looks like it goes away if you disable
connected textures (Options: Video Settings: Quality: Connected Textures: Off). So, no more pretty glass for a while.
Reloading the resource pack doesn't help and restarting the game only sometimes fixes it or (more likely) afflicts some other