Undiscoverable Features

Hold down △ when launching DESKTOP.SYSTEM to prevent DeskTop from being copied to a RAM card.
Desk Accessory files with high bit set in the aux type field will not appear in the menu.
You can't run a Binary file by double-clicking, but you can run it with the △-O shortcut or holding down △ or ▲ key while selecting File > Open.
The Sort Directory desk accessory has two modes:
- If any files are selected, these are moved to the start of the directory listing, in selection order; other files appear after, order unchanged.
- If no files are selected, all files are sorted by type: DIR, then TXT, then SYS, then others in descending numeric order.

Secrets and Mysteries

The Calculator desk accessory has a tiny monogram resembling "JB" drawn in the title bar - possibly "J. Bernard" thanked in the credits?
The string "Rien" appears in a couple of places in the code. Is this a name?
In the "This Apple" desk accessory, type E to see what other Apple models look like.