Strange behavior with os call in cgi script

[sophie_newbie]

I have written a cgi script that seems to run perfectly from the
command line when I simulate some cgi input using
os.environ['QUERY_STRING'].

The thing is that when I run it from the browser, one of my os.system
calls, which gets excecuted fine when running the program in the
interpreter, doesn't seem to get excecuted, or gets excecuted but does
nothing.

Does anyone know whats going on or how I could debug this problem?

It is worth noting that other os.system calls in the script get
excecuted fine.