os.execute() returning -1

[Francisco Santanna <fsantanna@...>]

From system() man page:
"If a child process cannot be created, or if the termination status for 
the command language interpreter cannot be obtained, /system/() shall 
return -1 and set /errno/ to indicate the error."
I get error -1 running os.execute() in the same big script, after a lot 
of other successful os.execute() calls.
Sometimes it just works and if I comment out  some lines before the 
error point it also works.
Am I missing something here? Any ideas where to look for? Maybe some limit?

Thanks.