Buying a Lemonade for 200 Pokédollars instead of 350 Pokédollars.
The vending machine purchase glitch is a glitch in Pokémon Red, Green, Blue, and Yellow.
If the player has enough money to afford a Fresh Water from a vending machine (200 Pokédollars) but does not have enough to afford a Soda Pop (300 Pokédollars) or Lemonade (350 Pokédollars), they can still buy the Soda Pop or Lemonade.
The player's money becomes 0 rather than a negative number.
The vending machine purchase glitch was documented by Háčky in 2015. It was also documented by the the Japanese "Kakeru" biglobe website.