Rationale:

The standard makes an embarrassing beginner's mistake.

Proposed resolution:

In [facet.num.get.virtuals], paragraph 7, add a row in the Length Modifier table to say that for double a length modifier l is to be used.

The table in paragraph 7 for the length modifier does not list the length modifier l to be applied if the type is double. Thus, the standard asks the implementation to do undefined things when using scanf() (the missing length modifier for scanf() when scanning doubles is actually a problem I found quite often in production code, too).