Minecraft – How quickly can a mob go back through a nether portal

You can put a single bucket of water into a dispenser. When a dispenser "dispenses" an empty bucket, it actually "picks up" a source block in front of it. By putting a single water bucket in a dispenser, when it's activated once it dispenses the water and when it's activated again it removes the water.

You can use this to make a button- or other restone device-triggered "off switch" for a portal. This is a complete reference design:

The trench behind the portal is what keeps the water from flowing everywhere:

To turn it back on, you can build a dispenser with Fire Charges directly against the frame. The Fire Charge goes through the frame, lighting it. The only problem with this is that it consumes a resource that has to be occasionally replenished. Thanks to Resorath for the tip and the video that demonstrates the technique:

This is most likely a rounding bug where the spawned portal in the nether, when trying to find a portal back in the over world, does the search but due to rounding and the 8 to 1 conversion rate, it will not find your original portal.

If you want a fix to this its the portal in the nether you want to destroy so that you can have it randomly generate a new one and give it a go. The problem is this means you have to hop to the nether, destroy the portal, store your gear and then die. When you go back into the nether your portal is going to be a bit of a ways away so make sure that wherever you store your gear it is -very- obvious to find it again.. Make sure you write down your coordinates and the like.

If hopping back into the portal to the over-world did not bring you to your original portal, it never will.

Please note that this rounding is potentially how to fast travel via nether hopping (hop into one portal and back out again to move 'forward' in the over-world).

Hope this helped.