For an AI course we had to write a solver for a puzzle using a heuristic. We had a reference solution step count to aim towards. After many hours of tweaking I still could not solve it. I had writ and rewritten quite complicated heuristics but to no avail, until at one point I stumbled on a nearly optimal solution. Upon reading my code again I noticed that a stray parenthes and a forgivinv lisp parser had caused most of my heuristic's code to be ignored. The simplest thing Just Worked :-)