Found this:

I can't say I'm dissapointed how plugin loading is treated in plosxom but
relying on alphabetical (or whatever) order of files in plugin directory
isn't good idea.

I set everything as the install.php and README asked me to. Everything
looked fine but to tell truth, I was quite surpised when I opened blog for
the first time with error message: "class 'standard' not found". Why should
be something dependent on file order when there's no guarantee of that. A
bit sick, isn't it? I thought out an idea how to fix that issue but thanks
to my lazyness I wrote one line into plosxom.php file:

include _once($config["plugin_path"]."/standard.php");

It's nothing horrible but it works. So after this all plugins could be
loaded properly. The end.

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Source]