Originally posted by jjrobinson2:

Portals will often malfunction if changed to a second address but entered before it switches.

Not a bug; it's a feature.

Being able to change the tag on a portal and step through to the original destination before it connects to the new one allows easier moving and renaming of portals.

Example:

I have portals at my home base that are named numbers (ie, "1", "2", etc) for disambiguation purposes, but I'm not sure where they will end up pointing to.
I find a good location for a new portal while exploring, which I decide will be named "Blueberries" for whatever reason.
When I build the portal, I name it "1", and wait for it to connect.
Then, I name it "Blueberries" and quickly step through, at which point I am transported to the "1" portal at my base... which I can now rename "Blueberries", wait a moment for connection, then step back through to the now properly named and connected remote portal and continue exploring.
When I find another good place for a portal, I can repeat the process for "2", and so on and so forth.