3. A surprising property or behaviour; in particular, one that is purposely inconsistent because it works better that way - such an inconsistency is therefore a feature and not a bug. This kind of feature is sometimes called a miswart.
4. A property or behaviour that is gratuitous or unnecessary, though perhaps also impressive or cute. For example, one feature of Common LISP's "format" function is the ability to print numbers in two different Roman-numeral formats (see bells, whistles, and gongs).
6. A bug that has been documented. To call something a feature sometimes means the author of the program did not consider the particular case, and that the program responded in a way that was unexpected but not strictly incorrect. A standard joke is that a bug can be turned into a feature simply by documenting it (then theoretically no one can complain about it because it's in the manual), or even by simply declaring it to be good. "That's not a bug, that's a feature!" is a common catch-phrase. Apparently there is a Volkswagen Beetle in San Francisco whose license plate reads "FEATURE".
The relationship among bugs, features, misfeatures, warts and miswarts might be clarified by the following hypothetical exchange between two hackers on an airliner:
A: "This seat doesn't recline."
A: "Oh. Then it's a misfeature; they should have increased the spacing between rows here."
"Undocumented feature" is a common euphemism for a bug.