This doesn't cause any problems when used normally, but it isn't best practice, and it means a number of bug reports coming up when I integrated it with phpBB3, which is how I spotted these two instances:
calendar.php
The print_monthly_view function (Line 402) uses $ME without defining it as a global, so it is using an undefined variable. It also means the URLs aren't correctly formed, but you get away with that. Adding it to the global variables fixes this (Line 404/405)
theme.php
Line 2286 tries to put (at least in some cases) an undefined "$results[($i + $row)]['events']" into "$events". This is fixed easily by checking that it is set before assigning it, for example replacing that line with the following fixes it:
$events = (isset($results[($i + $row)]['events'])) ? $results[($i + $row)]['events']: '';
I'll keep an eye out for any similar things. But I'm massively impressed with it! Love it! Exactly what I've been looking for. I'm currently working on integrating it with phpBB3, and if you're interested, I can give you all mod info when I'm done.