>> A comment "this CANNOT happen" has no value on itself.

> I think it does have some value: it makes clear an assumption the programmer made.

To me, a comment such as the above is about the only acceptable time to either throw an exception (in languages which support that construct) or otherwise terminate execution (such as exiting the process). If further understanding of the problem domain identifies what was thought impossible to be rare or unlikely instead, then introducing use of a disjoint union type capable of producing either an error or the expected result is in order.

Most of the time, "this CANNOT happen" falls into the category of "it happens, but rarely" and is best addressed with types and verified by the compiler.