In 2001, Pokemon the Movie 3 was released as a main feature and its pre-show was a short film called "Pikachu & Pichu". The short film involves Pikachu taking interest in a streetwise Pichu, getting separated from Ash and the gang in the process. From there other Pokemon start popping up and causing more trouble for Pikachu & Pichu as they explore - lots of antics.
The short ends with Pikachu reuniting with Ash and is surprised to see that the whole time they were planning a big surprise party. Pikachu is confused when brought to the party, and Ash explains that today was "the day we first met".
For those who can remember back to the first episode of the Pokemon anime, the day Ash became a Pokemon trainer was also the day after he turned 10 years old. And as we all know, Ash's first Pokemon was Pikachu! This makes the Pikachu & Pichu short into a strange writing error and fuel for countless fan theories; the writers of the show never wanted Ash to age, and while it's claimed by staff he's still 10, this is a scene where Ash openly describes the day in the short being the day he met Pikachu - the day after his own birthday. This should make Ash at the very least 11 years old! For years, it's left many fans scratching their heads, but it's a pretty funny detail if nothing else.