"Processing error message" is a predefined property containing a textual description of an error and is provided by Semantic MediaWiki. This property is pre-deployed (also known as special property) and comes with additional administrative privileges but can be used just like any other user-defined property.
Property "Early bird fee reduced" (as page type) with input value "{{{Early bird fee reduced}}}" contains invalid characters or is incomplete and therefore can cause unexpected results during a query or annotation process.
Property "On site fee reduced" (as page type) with input value "{{{On site fee reduced}}}" contains invalid characters or is incomplete and therefore can cause unexpected results during a query or annotation process.