why would chaining built in implicit rules not work?

[Mark Galeck (CW)]

Hello,

The manual says about one of "built-in" implicit rules:

"a file n is copied from n.sh"


I have a rule like this:

%.sh: %.sh.in
        cp $< $@


and then if a file foobar.sh.in is present,  and I need to update "foobar", 
then sometimes it works, and sometimes it says "no rule to make foobar.sh", and 
make fails.  Curiously, even when make fails, the file "foobar" is nevertheless 
updated correctly.  

I think I understand about .PRECIOUS, .SUFFIXES and I don't think these have 
anything to do with the problem.


Due to the intermittent and unpredictable nature of the problem, it happens 
only sometimes, not often, I have not been able to get a small reproducible 
example to show you.


So I am asking if anybody has seen that kind of problem before and what the 
solution was.


Thank you,

Mark