The rationale for providing information on processing errors via special property "Has processing error" is to make issues more visible and to provide better assistance for rectifying mistakes made. It gets automatically filled with a label for the "container" holding further information about the respective problem, i.e. the description of the error and if applicable further information like improper values or failed queries. Click on the search icon right next to the label for the processing error "container" to access it.
Special page "Processing error list" provides an easily accessible overview on processing errors and should be checked in regular intervals.
See special page "Processing error list" on this very wiki to see a list of processing errors.