What if you can't get 100% coverage?

Two possibilities:

• The code is dead (never used)
• You don't understand the system well enough

What are the drawbacks to this method?

• Tests are based on code, not requirements -- they can't tell you if you're missing something
• 100% has a clear meaning, anything less is up to judgement -- what's good enough?
• Most systems are too complex to get 100% Coverage
• Just because you've tested a function doesn't mean that you've tested all the code inside of it completely

From Udacity's Software Testing Unit 2