Let's say we need to use a shell/Perl/Python script which is covered by GPL. The script has a configuration section that it asks us to manually fill out, and let's say the section includes login information -- things like server hostname, username, password, and so on.
You know what? If we fill out those login information fields, they may become
modifications to the source code, and we may have to furnish them to whoever asks for them.
Whoa. I mean, a malicious script writer can release such a script, track the script usage, then later go like
I want your user ID and password! XD
I wonder if there's any legal device that prevents this from happening.
Edit: Apparently this is a moot concern... We are free to modify the source code for private use without releasing the source code or the binary compiled from it at all. Just modifying the source code doesn't trigger the GPL -- modifying the source code and distributing the resulting program does.