Dan Weinreb explains conditions:
When you design a function, you should first think of all the possible kinds of (correct) outcome. Then you should decide how each outcome will look to the caller: certain specific returned value(s), or certain specific conditions. This all becomes part of the contract for the function.
jwz reminisces about MOST-POSITIVE-BIGNUM:
Well, I was poking around in the system's basement one day, and realized that their implementation of bignums did have an upper limit! A bignum was implemented as an array, with no facility to tack on a second array, so the limit was related to the size of the length field in the array header (instead of being limited by available memory).
So I went and consed up MOST-POSITIVE-BIGNUM.
Over a couple of years we built a system consisting of eighty thousand lines of Lisp, having probably thrown away another fifty thousand along the way. We were in the classic situation described by Paul Graham in On Lisp: not knowing exactly what program we were writing when we set out on our mission.