Some of the most frustrating problems I've seen are due to misunderstanding or rare semantics rather than mistakes. Examples might include the specifics of floa

Some of the most frustrating problems I've seen are due to misunderstanding or rare semantics rather than mistakes. Examples might include the specifics of floating-point math, or exactly how numeric coercion works. This is especially bad when the result isn't easy to eyeball as right or wrong. For example, we could force a double-valued transcendental function to float and back for a small loss of precision in normal cases: #define sin(d) ((double)sinf(d)) Which yields... double i = 10 --> i = 10.0000000000 sin(i) = -0.5440211109 sinf(i) = -0.5440211296 delta = 0.0000000187 double i = 0.25 --> i = 0.2500000000 sin(i) = 0.2474039593 sinf(i) = 0.2474039644 delta = -0.0000000051 double i = 9999999.999999 --> i = 9999999.9999989998 sin(i) = 0.4205487007 sinf(i) = 0.4205478132 delta = 0.0000008875 (Format string was %25.10f, fwiw.) Another idea: Let's say we know the application relies on rand() to produce the correct distribution for testing, or maybe to generate session keys. Changing the semantics of the RNG won't result in a compiler bug, but it might result in the tests failing to cover all cases, or a horrible security breach.