Hello Stanislav,
I don't think that "invalid behavior" covers that sense: it means that implementation should actually do something to make code perform 'invalid' things (what should be considered invalid, by the way?), rather than have the possibility to best adapt the behavior to system (e.g. segfault) or some error handling mechanism (e.g. throw an exception).
Some where in the spec, D defines depending on order of evaluation to be invalid (placing any related bugs in your code, not the compiler, by fiat) but declines to requiter the compiler to enforce it (because it can't in many cases). Maybe some term for "invalid but un checked" should be used.
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