lucy24 - 11:56 pm on Nov 7, 2013 (gmt 0)
Memo to self: READ question before answering it.
If it's an SSI error, there should also be something in the html. By default it's a visible-to-the-naked eye text that says something like "SSI error" at the point where the include was supposed to happen. (If you don't knowingly use SSIs, you probably have the default error message.) A custom error message will probably be inside <!-- comment --> tags so you'll only see it in the page source, not in the visible page.
Worst-case scenario is if the server didn't even try to include the file; then the html will say the same thing the physical file says, beginning in <!-- #include. But in this last case, I don't think the server would even report an error, because it doesn't know that there was one.