Debugging
Defects where a diagnostic assertion encodes a condition the author believed unreachable, and execution reaches it anyway. These calls are compiled only into Debug builds, so the contract they express vanishes in Release — the violated invariant then passes silently instead of failing loudly.
Whether through Debug.Assert(false) or Debug.Fail, hitting one of these marks a state the code was never meant to enter. Treat it as a logic error that has already occurred, not merely a place that pops an assertion dialog under the debugger.