After successfully using portal pairs many times, the portal in the Nether just stops working. All the visual effects of the portal itself are still there, and I can still use a different portal to get back to the overworld, hike back to the first pair, and use it to get into the Nether, but you cannot use the Nether side to get back to Overworld on that pair. The wavy effects never start inside the portal, and teleport never happens.

This has happened now on two completely different worlds. I have not yet tried breaking and relighting the Nether side, though I assume that would work.