The ship Gemini 12 activated its unusual security system — a giant enveloping gas cloud which contained a "reality minefield". This consisted of temporary bubbles of "unreality" which would confuse and disorientate any potential looters of it's Time Drive, and deter them from delving deep into the stellar fog surrounding the Gemini 12.
The effects of these bubbles included making Dave Lister, a human crew member, appear to be a Series 3000 Mechanoid; making the Cat become invisible and removing his existence from the memory of the other crew members; making Starbug appear to disappear around the crew; and the heads of the crew transforming into the heads of animals.
As any ship penetrated deeper into the gas cloud, the effects of the "unreality bubbles" would become more and more unnerving, more ridiculous and more extreme. The crew counteracted the phenomena by going into the 'Bugs "Deep Sleep" where neither reality nor unreality could penetrate.
The unreality bubbles were mentioned again in the following episode, when the crew go to retrieve the Time Drive a second time after the battle with their future selves, and once again use Stasis to nullify the unreality. (Tikka to Ride, Series VII)