In Spyro: Year of the Dragon, there is a glitch where you may see two gems atop a tower which are unreachable by normal means in Fireworks Factory. This happens if your game's Automatic Difficulty Tuning (ADT) is set to easy. There is a code to undo this, and it is Circle, Square, Right, Left, Right, Square, Circle, X. This code sets your games' difficulty to medium. You can also press Square at the last entry instead of X, which sets it to hard. This glitch only happens on the PAL & non Greatest Hits versions of the game. The glitch was fixed in the Greatest Hits (US) version of the game.
Also, you may end up with 601/600 gems. It doesn't mean anything, just the game miscounting how many gems you have.
Another way to execute this glitch is to skip the Greta fighting cutscene as she is killing an enemy, this will cause the game to spawn 2 gems instead of 1 from the enemy.