Delay testing is a technique to determine if a chip will function correctly at a specified frequency. If a chip passes delay tests, it will presumably function at the specified frequency in the field. This paper presents experimental results that show how chips can pass very thorough delay tests and still fail in the field. It is shown that some chips sometimes pass and sometimes fail when the same delay test is applied multiple times under the same test conditions. These chips are called inconsistent fails. This paper shows how tester timing edge placement accuracy can cause inconsistent fails and suggests the minimum requirements for guardbands that avoid the inconsistent test results.