Blaine using a Super Potion on a Growlithe with full HP.
The Blaine Super Potion glitch is a natural glitch which occurs in Pokémon Red and Blue.
Blaine is able to use a Super Potion on one of his Pokémon even if it has full HP. This is unlike other Trainers who use healing items only if their Pokémon's HP is not full and it is below a certain ratio. This doesn't cause the HP of Blaine's Pokémon to be raised above the maximum; instead, it merely stays the same.
This oversight has been fixed in Pokémon Yellow, where it is impossible to have Blaine use a Super Potion on a Pokémon with full HP.
Different Trainer types have different AIs, detailed in this Pokémon Speedruns wiki article.
In Red and Blue, Blaine is able to use a Super Potion on one of his Pokémon even if it has full HP, as his Trainer AI routine does not check to see whether or not his current Pokémon has a given amount of HP before calling the function to use a Super Potion on it. In Yellow, his Trainer AI checks if his current Pokémon's HP is below 1/5 before calling that routine.