From Glitch City Laboratories
The Invisible Tree Glitch is a glitch where a tree that the player has just cut down becomes unable to stop the player as if it was not cut down and is not visible.
1. Go to Route 14, east of Fuchsia City.
2. Walk past a group of trainers, and the player should come upon a cuttable tree.
3. Cut down the tree and take four to five steps to the left.
4. Walk back to the tree. The tree will look as if it has been cut down but the player will not be able to walk through. It is possible to remove the tree by using Cut, making the tree reappear.