You are comparing a date column to a string literal. In such a case, Oracle attempts to convert your literal to a date, using the default date format. It's a bad practice to rely on such a behavior, as this default may change if the DBA changes some configuration, Oracle breaks something in a future revision, etc.
Instead, you should always explicitly convert your literal to a date and state the format you're using:
SELECT * FROM MYTABLE WHERE MYTABLE.DATEIN = TO_DATE('23/04/49','MM/DD/YY');