There comes a point, around twenty minutes into the show, when Pikachu uses his lightning attack to blow up some missiles. Because these are virtual missiles, and Pikachu is currently residing in Pokémon’s version of cyberspce, a regular explosion just wouldn’t look right.
So the animators used a rapidly-strobing technique that flashed red and blue lights on the screen, to make the explosion look “virtual”.
And then all hell broke loose. Read all about “Pokémon Shock”, a combination of the effects of strobe lighting combined with the sheer popularity of the program. It’s estimated that around 1 in 4000 people are vulnerable to “photosensitive seizures” and other health issues when viewing strobe lighting. That may sound minor, but when you consider over four million kids were watching that particular episode...
The original (now banned) sequence is below - watch at your own risk!