I've seen "100% coverage" specs that managed to run every line of code while only testing trivial, obviously correct, aspects and failing to test any of the actually important behavior. This sort of stuff gives TDD a bad name, but is, in my experience, sadly common.

Also, at least in the Ruby community, people seem to only measure line coverage. Without branch and conditional coverage reports, it's possible to run 100% of the lines but still miss a lot of code paths.