Or someone can send you a URL that points to your dev server and contains a payload. Or someone can send a URL to the developer across the street who uses shared hosting where display_errors is enabled by default.
I'm not arguing that this is a feasible attack to pull off against most applications: it most certainly is not. I'm arguing that it shouldn't be possible to pull it off at all, even in the small percentage of applications that might be affected. There is no good reason PHP should be displaying this information unsanitized.
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You're 100% correct about the example you're pointing out. If you're allowing arbitrary functions called from the user, you're in much bigger trouble. ;-)
I originally noticed and reported the issue with the first example, which triggers a simple, common undefined index notice:
<?php
error_reporting(E_ALL);
$array = array();
echo $array[$_GET['index']];