Define acceptence criteria for the program eg: must process square widgets sucessfully, and test, using every valid and invalid situation you can think of (does it reject triangles correctly?) until you meet *all* those criteria.
There's always going to be issues, you just need to be sure that the cost of resolving those issues will be within acceptable tolerances.
Of course, making sure your system is designed and specified properly in the first place will eliminate most defects and (IMHO) make the ones that show up just all that much easier to fix.
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*Unless* you never stop testing your program.
Define acceptence criteria for the program eg: must process square widgets sucessfully, and test, using every valid and invalid situation you can think of (does it reject triangles correctly?) until you meet *all* those criteria.
There's always going to be issues, you just need to be sure that the cost of resolving those issues will be within acceptable tolerances.
Of course, making sure your system is designed and specified properly in the first place will eliminate most defects and (IMHO) make the ones that show up just all that much easier to fix.
btw: http://www.fastcompany.com/online/06/writestuff.ht ml is a great article on how they test the software that goes into the Space Shuttle.