If you had 4 megabyte (or more) memory in your PC, items in your inventory would spin instead of standing still.
There appears to be four (4) possible endings to Rex Nebular:
Rex Nebular was prominently advertized as the first game to use the MADS engine (MicroProse Adventure Development System......SCUMM anyone?) used in other games like Dragonsphere.
Rex Nebular was the first game that Sid Meier protege Brian Reynolds worked on with MicroProse.
In addition to the already humorous manual the game originally came with Rex's "voice-activated auto-transcribing audio log" (actually a transcript of said log). This 20-plus page fiction gave a deeper insight into Rex, and described the events that led him to Terra Androgena in much more detail that in the opening sequence.
Written by Steve Meretzky, the log was also pretty funny to read, with highlights like reading an entry were he goes on and on about how exited he is about his new log and how he's gonna make an entry every single day, only to read that his next entry is a year later and reads "what the hell is this piece of junk?