User error:  This contains errors in the program (e.g., Logical errors) with errors made by online users of database.  These types of errors can be avoided by applying some check situations in programs or by limiting the access rights of online users' example, read only.  So only updating or insertion operation needs appropriate check routines that perform appropriate checks on the data being modified or entered.  In case of an error, some prompts can be passed to user to enable him/her to accurate that error.