It is a continuous struggle to understand how much a product should be tested before the delivery to the market. Ericsson decided to evaluate the adequacy of unit test coverage criterion that they employed for years as a guide for sufficiency of testing. Naturally one can think that if increasing coverage decreases the number of defects significantly, then that coverage measure can be considered a criterion for test sufficiency. To test this hypothesis in practice we investigated the relationship of unit test coverage measures and post-unit-test defects in a large commercial product of Ericsson. Based on the results we would like to indicate that the current unit test coverage measures do not seem to be any tangible help in producing defect-free software.