I found this quote on a software engineering book when I was doing my masters.
“Every bug found means there is yet another bug hiding in the product”.
I found this very true. In fact, I can extend it to “If you find n bugs, it means there are n more bugs to be found”.
Normally, in a testing cycle, you tend to find more and more bugs everyday for a period of time, reaches a peak and then starts declining.
So, above theory proves to be wrong at only one point i.e. at the peak of the curve.
This theory is a good guideline to measure the quality of your product.