I gave a Maths talk on the 100 Prisoners Problem and prepared a Powerpoint for the talk. On the opening slide, instead of writing the date, I noticed that the next day would be 14th March, or Pi Day, and so I wrote "The Day Before Pi Day".
A few months later in the same year I was asked to give the same talk. This was possibly only the second time I have given this talk formally to a large group. I decided to update the Powerpoint, and, naturally, this included the date.