I couldn't miss the chance to practice my drawing!
As glitch in the entrance
There is a glitch that may occur when obtaining the Save Crystal. When first entering the Northern Cave, the player can jump over and grab the Save Crystal before the rocks fall. This leads to the textbox saying the player got the Save Crystal remaining on the screen, but if one leaves the Northern Cave, returns, and checks their inventory, they will find they did not get the Save Crystal.
As bug in general
The item will create a save point anywhere in the final dungeon, but after a save point is used in several locations, and the game is reloaded, sometimes the player cannot leave that room, ruining their saved file permanently. The Save Crystal glitch often manifests in the room where the party splits up to find items and the swamp room where the enemy Mover appears.
Like many glitches in Final Fantasy VII, this bug may or may not occur for unknown reasons, and may occur in different locations. An example of the glitch working is if the item is used in the screen where the party is given the option to split up for the first time (the left and right paths). Using the Save Crystal here stops the player from exiting from the top (to travel towards the exit of the cave), and can even limit the player to a single path. Therefore, it is wise to ignore the crystal and travel through the cave without saving, or saving to a different file, or saving at the place where the party meets up for the final descent.
This glitch was not fixed in any subsequent rereleases.
How to copy the Save Crystal
This glitch allows the player to use the Save Crystal in the Northern Crater one time, creating a save point at a specific screen while keeping the actual Save Crystal item in the inventory allowing one to either keep the Save Crystal (for the purpose of a Perfect Game) or to create a second save point anywhere in the Northern Crater.
The trick is done in the third screen of the Left-Down path of the Northern Crater, and requires the player not to pick up the X-Potion from the chest there just yet. One needs to move close to the ledge of the first chest (which contains the X-Potion) allowing one to jump down and open the chest the split second Cloud starts jumping down. The player must not close the message that the player will have received an X-Potion. This message—if the player has done the above correctly—will not paralyze Cloud, allowing him to move close to the exit of the screen down below. Without exiting the screen, one must position Cloud in such a way his face is off the screen without actually exiting the area.
In this position the player can enter the menu and use the Save Crystal. When the game asks (in the field) if the player wants to create a save point here, the player should answer 'yes' and immediately run off the screen to the left. In the next screen (the bottom of the Northern Crater), the game should ask again to create a save point. Th player should answer 'no' and return to the previous screen. If done correctly, the player has created a save point, and still has the Save Crystal in the inventory.