Undocumented Hidden Parts
There are secret keys in Theta by Quad. When the intro asks you about your GUS port number, you may input "L" or "S" (in upper case). It turns on the Load data mode or Save data mode respectively. The intro loads or saves textures and music from/onto the disk. The 64K Erratic also has such feature, but I don't know how to turn it on (without a debugger).
When I debugged Sunflower by Pulse, I found there many commandline
switches, most of them described in the list of Demo Hidden Parts,
except one - it's the switch "-femeanine". With it, the demo writes a
hidden message to Sofia. I don't know who she is, maybe Unreal's
girlfriend?
And in Dive, a demo by UniVerse for Bytefall '99, there's a commandline switch "made", which makes the demo write a file called enter1.jpg with a picture to the disk.