Creating one nether portal in the overworld is spawning 2-3+ portals in the nether with no rhyme or reason as to which one you will spawn at and vice versa when traveling back. Sometimes spawning entire level 4 maps away from the portal youve come out of 100's of times. Traveling back from the nether will randomly spawn new overworld portals from time to time in locations that make no sense. 

Spawning in the nether inside walls, dying and losing all your progress/items from overworld portals youve used numerous times while having built no new ones. Getting suffocation damage will spawning into the nether when the portal/room is completely created for safe portal travel. 

My friend and I have lost weeks of progress due to the portals being messed up and we cannot progress in the game as going to the nether is just too broken.