Why 100% Code Coverage Isn’t Optional Anymore

Software developers tend to focus on the “happy paths’—the main use cases. Just covering them gets you to maybe 50–60%. Next come the edge cases and error handling. That might push you to the industry’s magic number of 75%, or 85% if you're especially thorough. But that last 15% is where bugs hide.

Imagine you're in a hospital.

You’re lying in a room, waiting to receive treatment with a brand-new, high-tech medical device. A technician wheels it in—sleek, modern, fresh off the assembly line.

While setting it up, they casually mention:

“This device just came in—first time being used in the real world!”

And then:

“Our engineers tested about 85% of it before shipping. Pretty solid, right? You’ll be fine.”

Would you let them continue?

Probably not. You’d ask them to stop, and you ...