Fault-tolerantThe term for the ability of a system to continue normal operation despite the presence of hardware or software faults.
Traditionally, this is accomplished through various kinds of redundancy, but for dynamic programming languages (those which can be debugged live at the level of source code), there is often the possibility of resuming in the face of an error or a lack of information. Common Lisp's condition system is just such a recoverable system, and this may be done automatically.
More expressiveness of the language in its higher-order constructs can help the system perform more comprehensive analyses of the situation before proceeding. (TODO: write up some examples illustrating this point technically.)