Quite a few of the bugs we see in production systems come from data that isn't handled well. Perhaps the developer never considered this data, or another bug lets data into a system that should never be recorded. These are often NULL values, but they could be other data values that are far out of the ordinary.

Where do we draw the line for edge cases? Is it anything that doesn't fit 95% of the data range? I see this number used in many fields, often manufacturing and other "physical endeavors". Is it the 80% rule, where we ensure 80% of data cases are covered, but 20% represent special handling?

Read the rest of What's the Edge Case?