According to
http://hg.openjdk.java.net/jdk/jdk/file/2fbc66ef1a1d/src/hotspot/share/runtime/globals.hpp#l38

// develop flags are settable / visible only during development and are constant in the PRODUCT version
// notproduct flags are settable / visible only during development and are not declared in the PRODUCT version

which seems to suggest that using a nonproduct flag in a product build would result in a compiler error of non-declared variable.

However, in the bottom of globals.hpp, nonproduct flags are declared in product build anyway. There doesn't seem to be any practical differences between nonproduct and develop flags.

#ifdef PRODUCT
#define DECLARE_DEVELOPER_FLAG(type, name, value, doc) const type name = value;
#define DECLARE_PD_DEVELOPER_FLAG(type, name, doc) const type name = pd_##name;
#define DECLARE_NOTPRODUCT_FLAG(type, name, value, doc) const type name = value;

For simplicity, we should change all nonproduct flags to develop flags.