Knights and knaves

There is a wide variety of puzzles about an island in which certain inhabitants called "knights" always tell the truth, and others called "knaves" always lie. It is assumed that every inhabitant of the island is either a knight or a knave...

Suppose A says, "Either I am a knave or else two plus two equals five." What would you conclude.?

If A is a knight, then two plus two equals five, which is not true. If A is a knave, then he is speaking the truth, which is not possible.

Smullyan comments:

The only valid conclusion is that the author of this problem is not a knight. The fact is that neither a knight nor a knave could make such a statement.