You can enter impossible mode several times

Yes, I know the impossible difficulty level is just a joke, but...

>IMPOSSIBLE
FC: Okay, you asked for it...

FC INTERRUPT: External sensors detect huge radiation abnormalities in the star
which provides Contra with all light and heat.

WARNING! TIME CRITICAL!!

External sensors detect significant instability in the star.

>IMPOSSIBLE
FC: Okay, you asked for it...

FC INTERRUPT: External sensors detect huge radiation abnormalities in the star
which provides Contra with all light and heat.

WARNING! TIME CRITICAL!!

External sensors detect significant instability in the star.

That's because it doesn't actually check if you're already in impossible mode. Compare this to how it responds to "ADVANCED":

>ADVANCED
FC: Request for advanced game acknowledged.

SENSA INTERRUPT: Secondary tremor detected by Filtering Computers. Intensity:
8.4. Projected damage: Automatic controls for surface transportation; Automatic
controls for Hydroponics Area.

IRIS: In the Weather Monitors.
WALDO: In the Gamma Repair.
SENSA: In the Central Chamber.
AUDA: In the Entry Area.
POET: In the Central Chamber.
FC: Whiz is no longer in communication.

>ADVANCED
FC: You are already in the advanced situation.

Note that you can't abuse this bug to actually win impossible mode, because you can still only enter it on the first turn of the game.

I think the correct behavior for this wold be to only allow you to go to a higher difficulty, i.e.:

Normal to Advanced should be fine.
Normal to Impossible should be fine.
Advanced to Impossible should be fine.
Advanced to Advanced should be blocked.
Impossible to Impossible should be blocked.
Impossible to Advanced should be blocked.

the-infocom-files / suspended

You can enter impossible mode several times #26