The documentation says:

""
 * Throws:
 *     A $(LREF ConvException) if `source` is empty, if no number could be
 *     parsed, or if an overflow occurred.  
""

The following float literal causes libc strtod to signal overflow inerrno:

   "999123254986799969899949354352145897441435999878464164684643513213254364543545634563454199999999999999999999999433212313213119992313453124136468463543543543999999341534654646546464646541341353541999999999965157349999999999320135273486741354354731567431324134999999999999999999999999999999999999999999999135411.9"

But std.conv `to` and `parse` accept it without throwing

---
void main()
{
    import std.conv;

    auto src = "999123254986799969899949354352145897441435999878464164684643513213254364543545634563454199999999999999999999999433212313213119992313453124136468463543543543999999341534654646546464646541341353541999999999965157349999999999320135273486741354354731567431324134999999999999999999999999999999999999999999999135411.9";
    auto cv1 = parse!double(src);
    auto cv2 = to!double("999123254986799969899949354352145897441435999878464164684643513213254364543545634563454199999999999999999999999433212313213119992313453124136468463543543543999999341534654646546464646541341353541999999999965157349999999999320135273486741354354731567431324134999999999999999999999999999999999999999999999135411.9");
}   
---