Just because there's code coverage doesn't mean you're actually testing all paths through the function.
For example, this code has four paths:
if (A) { ... } else { ... }
if (B) { ... } else { ... }
However just two tests (e.g. one with A and B true, one with A and B false) would give "100% code coverage."
This is a problem because the tendency is to stop testing once you've achieved the magic 100% number.
