I'm not sure what's going on here. I'm trying to _ss_environment.php files for a first project and the following is being echoed before any other content is generated, thus throwing up XML parsing errors and muddying /admin/ and dev/build.

Here's the list that's returned:

<ul><li><b>Director::set_environment_type(dev/test/live)</b>
<br />
Line 56 of ConfigureFromEnv.php</li>
<li><b>require_once(C:\path_to_silverstripe\sapphire\conf\ConfigureFromEnv.php)</b>
<br />
Line 8 of _config.php</li>
<li><b>require_once(C:\path_to_silverstripe\mysite\_config.php)</b>
<br />
Line 6635 of manifest-main</li>
<li><b>require_once(C:\Windows\Temp\silverstripe-cacheC--formatted_path_to_silverstripe\manifest-main)</b>

<br />
Line 70 of ManifestBuilder.php</li>
<li><b>ManifestBuilder::include_manifest()</b>
<br />
Line 150 of Core.php</li>
<li><b>require_once(C:\path_to_silverstripe\sapphire\core\Core.php)</b>
<br />
Line 61 of main.php</li>
<ul>

This sure LOOKS like an error report, but where is it coming from? And what's with manifest-main requiring a file from my Windows cache? Shouldn't this sort of data be stored somewhere in Apache?

And the problem was in _ss_environment.php
Make sure that define('SS_ENVIRONMENT_TYPE', 'dev/test/live'); is only passed the one of the three strings, like so:
define('SS_ENVIRONMENT_TYPE', 'dev');
define('SS_ENVIRONMENT_TYPE', 'test');
define('SS_ENVIRONMENT_TYPE', 'live');

facepalm