Variable record format
Hehe... Blast from the past - full OS level record management in file IO. The app developer had no clue what is going on behind the scenes, VMS was managing it all for them including by default revisioning the file on each open for write. So if it decided to do the actual writes as variable size, the app developer would have had no clue of that - it would have looked like ordinary record retrieval to the application.
The end result was the most insane open() syntax known to man. My recollections were that it took 5+ lines of optional args to open a file in VMS Pascal.