Syntax error populating listbox

For an application I am helping to develop I have forms that have a listbox at the top of the form containing parent records and then many listboxes on tabs at the bottom of the form that contain the child records.  I create cursors to populate the listboxes for the child
records and set the rowsourcetype = 6 and rowsource = "table1.field1,field2, etc..."  Everytime the user selects a parent record I rebuild the cursors to contain the correct data.  On very rare occasions the program crashes on the line rowsource = "table1.field1,field2, etc ..."  and gives the error "10 - syntax error".  I've checked to make sure the that line is okay and it is.  I have never been able to duplicate this error when I am trying to debug it but the error appears every once in awhile.  Has anyone ever seen this?  Any ideas why this happens?  It concerns me that this error just pops up on rare occasions and I am unable to duplicate it.