During an upgrade, ADODB SQL statements may produce warning messages, e.g. if you update BIGINT column by DATESTAMP value from another column (this happened to me today during 1.9 beta upgrade). The page is not rendered, even if an upgrade is successful.
Example of produced warning:
<pre align=left> ... </pre>
Dunno if this is in Moodle tracker scope but it is a little bit confusing.