What are the most common Doomsday mistakes?

Forgetting the leap-year shift for Jan/Feb, counting the offset the wrong direction, and fumbling the mod-7 wrap.

• Leap-year slip: using Jan 3 / Feb 28 in a leap year (should be 1/4 and 2/29). Always check leap status first for Jan/Feb dates.
• Direction error: the target can be before or after the doomsday date; subtract accordingly and allow negative offsets.
• Mod-7 wrap: remember 0 = Sunday, and that Tuesday − 3 wraps to Saturday (6), not a negative day.

Every one of these is caught by a quick self-check against a date whose weekday you already know.