Description:
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It's only happened once, but it's a little scary considering 
it caused a fatal error for the page (without a chance of 
recovering on it's own).

When a script was run, it threw an error about a call to an 
undefined function "impkode" on a certain line.  I checked 
the file, and there was no imkode() function, but there was 
an implode() function.

The PHP file itself was not edited and the file system shows 
it not being edited in over 6 months.

The only thing I did to fix it was the "Clear opcode Cache" 
function in apc.php

Might not be a bad idea to do a sanity check on the 
compilation (even as an optional parameter).  Like maybe 
compile PHP files twice to make sure they the same both 
times or something.