Here's one purely for fun. I was looking for a February 23rd cover to post here (didn't find one) but I did come across this so I thought I'd include it.
What's wrong? Can you see it?
Answer: The date is shown to be February 23, 1980 but if you look at the catalogs, the correct day of issue should be February 25, 1980. That also goes along with the fact that the cachet even notes the celebration of "National Letter Writing Week" (do we have that anymore?) from February 24 to March 1, 1980, so it wouldn't make sense that the stamps were issued before that date.
Upon closer inspection, the day of issue is correct at February 25th. It's just that the postmark hit the stamp right at its bottom perforation and created the optical illusion that the "25" was a "23".
Sure had me fooled for awhile until I enlarged it to see the true date!