ToME: the Tales of Maj'Eyal

When I stepped over a naga invasion portal, it didn't prompt me to close or enter it. I hit the "go down stairs" key anyway, and it brought me inside the portal. I killed the rare inside, but when I got out the portal was still open. This time, I stepped over it and it did prompt me to close or enter. I chose enter, and it threw a lua error, but I forgot to grab it. Stepping off of the portal, then stepping back on and choosing destroy, destroyed it properly.

Now that I think about it, I'm willing to bet that what happened was that I switched places with a summon who was standing on the portal, and that failed to trigger the enter/destroy dialogue box.