I had a creeper explode next to my nether porthole in the overworld, knocking out the two lower blocks of porthole. This is supposed to fully deactivate the porthole but it did not; the 4 upper blocks were still active. I could still step into it and be transported to the nether, however, whenever I return, I take damage. Attempting to re-light the porthole with flint and steel just makes fire in the lower, empty spaces.