It was likely a glitch. Like the comments said, it is what one of the trainers say after you beat them.
But this is disproven, because Yellow, being a Gen 1 Game, has absolutely no glitches and is the best game in the franchise. (This sentence above was sarcasm.)
So, in summary, it was was definitely a glitch, considering that it was Yellow, one of the most glitched games in the franchise.
Hope I helped!
Also, thanks Blaziken for his image in his answer!