Believe the terrain, not the map
An effective way to test code is to exercise it at its natural boundaries
Get the weirdnesses into the data where you can manipulate them easily, and the regularity into the code because regular code is a lot easier to work with
Don't document bad code - rewrite it.
Each new user of a new system uncovers a new class of bugs.
... it is a fundamental principle of testing that you must know in advance the answer each test case is supposed to produce. If you don't, you are not testing; you are experimenting.