Caustic window was a vinyl pressing from 1999 that Mike Pardinas (µ-ziq) described in a rare interview. It's an acid houseish album by Aphex Twin.
Only 4 people at the time of the interview were known to have a vinyl test pressing of the album - Richard James, Chris Jeffs (aka Cylob), Grant Wilson-Claridge (co-founder of Rephlex Records) and Mike himself. Each person was sworn to never make copies of the music, and for 20 years, not a single track had ever been commercially released (with the exception of two tracks that appeared on compilations).
Now it's been released however, and it's pretty cool. Listening to it in order right now, it seems the songs get more hectic and wild as the album go on.
The full album has been uploaded to youtube here.
My opinion is that this really isn't his best stuff, it's a bit out there(Then again, I dont think there's any of his stuff that isn't), but it's pretty cool. so far(listening to this while I write this) I like Fingertrips the best.