If you require() or require_once() on a file that's not found anywhere in the library search path, PHP gives no output and no error message.  

This is so common in our shop that whenever we see no output at all from a script, we just assume we've goofed up a require().  When I stumbled across the behavior in Bug #13347, I figured I messed up an require().

It may also be a problem with include() and include_once(), but we don't use those anywhere anyway.