I ran into an issue with the Commenting plugin, where any comment would create an error and subsequently would not get posted. This was caused by the parent_id being left empty, and the framework throwing an error, instead of defaulting to add a null value. The workaround has been documented in the Google group - see here - http://bit.ly/HEEOeq
Now it has been suggested that it was caused because my MySQL install is running in strict mode, but as far as I can tell this is not the case.