When viewing some reports (stats based) there are some invocations to reportsgraph.php that, produce the error:

Fatal error: Call to undefined function: imagecreate() in moodle_17_stable/lib/graphlib.php on line 1415

I think it will be smarter to check for GD availability, just like some activity logs page are doing currently, showing the message:

(GD must be installed to see this graph)