Many early Common Lisp systems had impressive error handling. Certain exceptions would automatically break to an interactive Lisp window. But Common Lisp also

Many early Common Lisp systems had impressive error handling. Certain exceptions would automatically break to an interactive Lisp window. But Common Lisp also has a fancy system for recovering from exceptions. So after you replaced the broken function, you'd often see a list of "restart" points from where you could retry a failed operation. So even if you failed in the middle of a complex operation, you could test out a fix without needing to abandon the entire operation in progress. I'm not entirely certain if this was as useful as it sounds, in practice, but it was certainly very impressive.