server log at debuglevel 3 has the following entries :

4117:20100811:150038.643 Format error or unsupported operator. Exp: [ab]
4117:20100811:150038.643 Format error or unsupported operator. Exp: [ab]
4117:20100811:150038.643 Item [A Test Host:a] error: Format error or unsupported operator. Exp: [ab]

first two are redundant and miss important information (host and item name), so they probably should be removed