I've just noticed this weird bug (in 1.7) and posted bug report MDL-8553.
If anyone can confirm it, please vote for the bug. Using Moodle 1.7+
Create a Cloze type question with one numerical sub-question in it, like this:
Moodle has a large and diverse user community with over 100,000 registered users on this site alone, speaking over {1:NUMERICAL:~%100%70:10#OK} languages.
Save and put this Cloze question into a Quiz called e.g. my_quiz.
Do a course backup, backing-up only my_quiz.
Do a course restore.
Course backup goes OK.
Preview quiz my_quiz or go to Admin/Questions and try to preview or edit the cloze question and you get this error message:
Error: Missing question answer!
Unable to get options for questiontype numerical (id=15)
However, if you go into Edit question mode for that faulty question, its full code is still there, just save the question and ... all goes well!
NOTE.- If, instead of doing a course backup and restore operation with my_quiz you simply do an export as file (Moodle XML) followed by a file import, all goes well, no problem.
Weird, ain't it? 
Not that is bothers me much, as I rarely use numerical questions myself, but I know others do.
Joseph