The world will be different depending on where you come from.

I found this when exploring some seeds. After revisiting a seed i noticed the world was different. Most notable was the monster spawner witch may or may not exist depending on how you got to it.
There appear to be 3 states:
1. There is no dungeon.
2. There is a dungeon without a monster spawner
3. There is a dungeon with monster spawner

Steps to Reproduce:
Step 1: create a world using the seed: -1955892699
Step 2: teleport to x=-715, y=18, z=-150.
There will be a dungeon with monster spawner.

Step 3: recreate the world using the seed: -1955892699
Step 4: teleport to x=-915, y=180, z=-150.
Step 5: fly/walk and dig to x=-715, y=18, z=-150.
There there will be a stronghold hallway.

Step 6: recreate the world using the seed: -1955892699
Step 7: teleport to x=-715, y=180, z=-450.
Step 8: fly/walk and dig to x=-715, y=18, z=-150.
There there will be a dungeon without a monster spawner.

Note! The version needs to be the same in order for this to work 1.7.10. If it isn't the stronghold itself may also change.