I posted about this on Stack Overflow thinking this was a general PHP problem, but I'm beginning to think this is an issue specific to Phalcon. [Stack Overflow thread]
PHP Fatal error: Uncaught exception 'Phalcon\\Mvc\\Model\\Exception' with message 'Unknown expression' in /home/acarbonaro/testApplication/index.php:84\n
Stack trace:\n
0 [internal function]: Phalcon\\Mvc\\Model\\Query->_getExpression(274, false)\n
1 [internal function]: Phalcon\\Mvc\\Model\\Query->_prepareInsert()\n
2 [internal function]: Phalcon\\Mvc\\Model\\Query->parse()\n
3 [internal function]: Phalcon\\Mvc\\Model\\Query->execute(Array, NULL)\n
4 /home/acarbonaro/testApplication/index.php(84): Phalcon\\Mvc\\Model\\Manager->executeQuery('INSERT INTO Pro...', Array)\n
5 [internal function]: {closure}()\n
6 /home/acarbonaro/testApplication/index.php(115): Phalcon\\Mvc\\Micro->handle()\n
7 {main}\n
thrown in /home/acarbonaro/testApplication/index.php on line 84