In the Star Wars universe, there exists a carbon freezing process that has been used before to transport frozen gas over long distances. In Cloud City, the carbon freezing process is used for the first time to encase a living person in suspended hibernation.
When Leia rescues Han Solo from his carbonite prison, Leia comments on the onset of hibernation sickness that Han is experiencing:
Leia: "Just relax for a moment. You're free of the carbonite. Shh. You have hibernation sickness."
Han: "I can't see."
Leia: "Your eyesight will return in time."
However, earlier, Lando Calrissian comments on how carbon freezing has never (to his knowledge) been used on humans before:
"Lord Vader, we only use this facility for carbon freezing. If you put him in there it might kill him."
How does Leia know that Han is experiencing hibernation sickness from being frozen in carbonite, if Han is the first person in history to go through that process? Even if hibernation sickness is not exclusive to carbon freezing, how does Leia know that it is specifically what Han is experiencing?
'we only use ***this*** facility for carbon freezing'doesn't seem to imply that no facilities exist in that could be used for carbon freezing life forms. After all C-3PO seemed pretty confident that if Han survived the freezing he should be perfectly safe. How would C-3PO possibly have known that, if it hadn't been done before.