An attempt was made to perform an operation on incompatible datatypes. For example, adding a character field to a date field (dates may only be added to numeric fields) or concatenating a character field with a long field.
An attempt was made to perform an operation on a database object (such as a table or view) that is not intended for normal use. For example, system tables cannot be modified by a user. Note that on rare occasions this error occurs because a misspelled object name matched a restricted object's name.
An attempt was made to use an undocumented view.
Source : AllAboutOracle.com
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