Incompatible Data

Except for the class condition (see the topic Class Condition), when the contents of a data item are referenced in the Procedure Division and the contents of that data item are not compatible with the class specified for that data item by its PICTURE clause

ANS85 or function definition

then the result of such a reference is undefined.

MF  The results of referencing a numeric field that contains nonnumeric, or otherwise invalid data, are undefined. Such conditions may be detected, and give an error at run-time. This behavior is affected by the F RTS switch.

MF When an alphabetic field which contains non-alphabetic data is referenced, execution continues, but results may be undefined.