The Book of Common Prayer contains the confession
… we have left undone those things which we ought to have done, and we have done those things which we ought not to have done.
The things left undone are called sins of omission; things which ought not to have been done are called sins of commission.
In software testing and debugging, we focus on sins of commission, code that was implemented incorrectly. But according to Robert Glass, the majority of bugs are sins of omission. In Frequently Forgotten Fundamental Facts about Software Engineering Glass says
Roughly 35 percent of software defects emerge from missing logic paths, and another 40 percent are from the execution of a unique combination of logic paths.
If these figures are correct, three out of four software bugs are sins of omission, errors due to things left undone. These are bugs due to contingencies the developers did not think to handle. Three quarters seems like a large proportion, but it is plausible. I know I’ve often written plenty of bugs that amounted to not considering enough possibilities, particularly in graphical user interface software. It’s hard to think of everything a user might do and all the ways a user might arrive at a particular place. (When I first wrote user interface applications, my reaction to a bug report would be “Why would anyone do that?!” If everyone would just use my software the way I do, everything would be OK. )
It matters whether bugs are sins of omission or sins of commission. Different kinds of bugs are caught by different means. Developers have come to appreciate the value of unit testing lately, but unit tests primarily catch sins of commission. If you didn’t think to program something in the first place, you’re not likely to think to write a test for it. Complete test coverage could only find 25% of a projects bugs if you assume 75% of the bugs come from code that no one thought to write.
The best way to spot sins of omission is a fresh pair of eyes. As Glass says
Rigorous reviews are more effective, and more cost effective, than any other error-removal strategy, including testing. But they cannot and should not replace testing.
One way to combine the benefits of unit testing and code reviews would be to have different people write the unit tests and the production code.