Lisping at JPL by Ron Garret, 2002 (must have read this at least a couple of times previously, but recently it popped up on HN - not for the first time.)
The Remote Agent software, running on a custom port of Harlequin Common Lisp, flew aboard Deep Space 1 (DS1), and controlled DS1 for two days in May of 1999. During that time we were able to debug and fix a race condition that had not shown up during ground testing. (Debugging a program running on a $100M piece of hardware that is 100 million miles away is an interesting experience. Having a read-eval-print loop running on the spacecraft proved invaluable in finding and fixing the problem. The story of the Remote Agent bug is an interesting one in and of itself.)