Can you test something too much? Sure.
If you spend all your time testing FeatureA and four other Features go untested, you’ve probably tested FeatureA too much.
Part of what makes some testers better than others is their ability to know how much testing effort to put into each Feature and when. This is a tricky decision and may require the tester to stop testing FeatureA, even though FeatureA is still yielding bugs. What?!! Think about it. If you know FeatureA is yielding bugs but you don’t know anything about FeatureB, where should you spend your time next? I say, FeatureB. It just feels wrong, doesn’t it?
A lot of testers have a hard time with this. They get wrapped up in completing exhaustive testing on one Feature while the unknowns sit and collect dust. They follow their nose for the reward of bug discovery in comfortable areas. They underestimate how much time it will take to test the unknowns and before they know it, opps, not enough time to test everything!
Have you ever seen one of those plate spinning circus acts? You know the ones, the guy is frantically running around trying to keep all the plates spinning on sticks so they don’t fall. As a tester, you should be like a plate spinner. The Features are the plates. I don’t know what the sticks are, don’t worry about it. But if your programmers are working on Features (e.g., fixing bugs, finding bugs, refactoring, testing), your plates are spinning.
Keep your plates spinning!