mi pu viska re prenu means "I saw only two people; I saw no more than two people and no less than two people". It does not mean "I saw two people".
This is the Goatleg Rule, so called because the point was originally exemplified by an example demonstrating that re da tuple lo'e kanba is false.
See the Book, chapter 6, section 6: "In Lojban, you cannot say 'I own three shoes' if in fact you own four shoes. Numbers need never be specified, but if they are specified they must be correct."
Any English speaker claiming "I saw two people" when it is not true that "I saw only two people; I saw no more than two people and no less than two people" will be accused of lying, or at least violating conversational maxims. (Except in specific cases where both parties are aware of an implicit "at least") --jay
It means "I saw exactly two people during some unspecified time interval in the past". Not much more confusing than the English. — Adam
So...where's the gotcha? Isn't 2 = 2 in English too? --xod
The Gotcha is that Lojban here does behave like English, and natural languages in general. In strict logical terms, when you see three people, it is true to say that you saw two people. (In fact, "2=2" is a defeasible implicature even in English: I saw two people... in fact, three.) Because this is uncooperative speaker behaviour, however, this has been judged inappropriate for Lojban. Early victory for Pragmatics! — nitcion.