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There are several potential causes for the getDay() method to return an incorrect weekday:

1. Invalid input: If the input date is not a valid Date object or contains invalid values for day, month, or year, then getDay() may not return the correct weekday.

2. Timezone issues: If the input date is in a different timezone than the system timezone or the browser timezone, getDay() may return a different weekday than expected.

3. Date format: Depending on the format of the input date string, getDay() may not correctly extract the day, month, or year values, resulting in an incorrect weekday.

4. Leap year issues: If the input date falls on a leap year, the getDay() method may return an incorrect weekday since the number of days in February differs from a non-leap year.

5. Overlapping timezone: If the input date falls on a DST transition day where the local time overlaps with a different timezone, the getDay() may return an incorrect weekday due to the ambiguity of the local time.