This mod was originally designed for use in adventure maps. As of December 26th, 2012, the Portal Gun Mod has been re-written to be balanced for use in survival Minecraft. Find more information about the re-write here.
There is an Easter Egg in the mod where if you shoot a portal at the moon and then shoot another portal anywhere else, it will create a vacuum that will suck in any blocks and players nearby. A player in survival mode will die similar to how a player would die in the void. Blocks and items sucked into the portal are lost, including items in a player's inventory. In creative mode, this portal teleports the player to y = 400m.